Grammar Conjunctions LexFizz Team 9 min read
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Key Takeaways
  • Time clauses tell us when an action happens in relation to another action.
  • The critical rule: after a time conjunction, use a present tense (not will) to talk about the future.
  • When = at the moment of / on that occasion; while = during an ongoing activity.
  • Until = up to a point in time; by the time = before a deadline or reference point.
  • As soon as = immediately after; it emphasises speed or urgency more than when.

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Time clauses are subordinate clauses that describe when something happens. They are introduced by time conjunctions such as when, while, as, before, after, until, as soon as, and by the time. Choosing the right conjunction — and using the correct tense in each part of the sentence — is one of the most reliable ways to demonstrate grammatical sophistication in English.

What Are Time Clauses?

A time clause is a dependent clause that cannot stand alone as a sentence. It must be attached to a main clause. Together, they form a complex sentence that expresses the relationship in time between two events.

Main clause: "I will call you."

Time clause: "when I arrive."

Together: "I will call you when I arrive."

When the time clause comes first, it is followed by a comma:

Time clause first: "When I arrive, I will call you."

Main clause first: "I will call you when I arrive." (no comma)

The Eight Main Time Conjunctions

WHEN At the moment of / on that occasion

When refers to a specific point or moment in time. It can describe a single completed event or introduce habitual/repeated actions.

  • "When I was a child, I lived in Spain." (past habitual)
  • "When she arrives, the meeting will begin." (future: use present simple after when)
  • "When he opened the door, the cat ran outside." (two past completed actions)

WHILE During an ongoing activity

While describes two actions happening at the same time, at least one of which is ongoing (in progress). Use a continuous tense in the while-clause when emphasising the ongoing nature.

  • "While I was cooking, the phone rang." (interrupted activity)
  • "While she was reading, he was watching TV." (two simultaneous activities)
  • "While I agree with your point, I think there are other factors." (concession — formal)
When vs While: Use when for a specific moment; use while for a duration. "When the phone rang, I was eating." vs "While I was eating, the phone rang." Both are correct here, but while emphasises the ongoing activity.

AS At the same moment / as time passes

As is used for two actions happening simultaneously, or to describe gradual change over time.

  • "As he walked into the room, everyone fell silent." (exactly at the moment)
  • "As I get older, I appreciate simple things more." (gradual parallel change)
  • "As the years passed, she became wiser." (ongoing parallel progression)

BEFORE Earlier than another action

Before indicates that the action in the main clause happens prior to the event in the time clause.

  • "I always check my mirrors before I drive." (habitual)
  • "She finished her homework before she went out." (past)
  • "Make sure you read the instructions before you start." (imperative + future)

AFTER Later than another action

After indicates that the main clause action happens following the time clause event. The past perfect can be used in the time clause to emphasise that one action was fully completed before the next began.

  • "After we had eaten, we went for a walk." (past perfect + past simple)
  • "After she finishes work, she usually goes to the gym." (present simple for habit)
  • "After you submit the form, you will receive a confirmation email." (future)

UNTIL Up to a point in time

Until describes the duration of an action or state up to a specific point. The action continues right up until the moment indicated.

  • "I waited until she arrived." (the waiting ended when she arrived)
  • "Don't leave until I get back." (imperative with future meaning)
  • "He won't stop talking until someone interrupts him." (continuous situation)
Until vs By the time: Until describes continuous duration up to a point. By the time means "at or before the point when". "I will wait until he arrives" = I wait the whole time. "By the time he arrives, I will have left" = I will have already gone.

AS SOON AS Immediately after / the moment that

As soon as emphasises that the second action happens immediately after the first, with no delay. It is more emphatic than when.

  • "As soon as she heard the news, she burst into tears." (immediate reaction)
  • "I'll let you know as soon as I have any information." (future: present simple after as soon as)
  • "As soon as you press Save, the document will be uploaded." (instructions)

BY THE TIME Before a reference point in time

By the time is used with the future perfect to say that one action will be completed before another action or moment occurs.

  • "By the time you read this, I will have already left." (future perfect)
  • "By the time we arrived, the film had already started." (past perfect)
  • "By the time they fix it, we will have found another solution." (future perfect)

Tense Rules in Time Clauses

The single most important tense rule for time clauses is this:

The Golden Rule
  • When talking about the future, use a present tense (not will) after a time conjunction.
Incorrect (common mistake)Correct
"Call me when you will arrive.""Call me when you arrive."
"I'll start as soon as he will be ready.""I'll start as soon as he is ready."
"Don't go out before it will stop raining.""Don't go out before it stops raining."
"She'll send the file after she will check it.""She'll send the file after she checks it."

The present simple is the most common tense in future time clauses. The present perfect can also be used to emphasise that the time-clause action is fully complete before the main clause action:

Simple: "I'll email you when I finish." (general sequence)

Perfect: "I'll email you when I have finished." (emphasises full completion before emailing)

Common Mistakes

Top 5 Time Clause Errors

  1. Using will in the time clause: "I'll call you when I will arrive" → "I'll call you when I arrive."
  2. Confusing when and while: Use when for a point; while for a duration. "When I was sleeping, I heard a noise" should be "While I was sleeping…"
  3. Missing the comma when the clause comes first: "When I get home I will cook dinner" should have a comma after home.
  4. Confusing until and when: Until expresses duration; when expresses a point. "I waited when she arrived" should be "I waited until she arrived."
  5. Overusing by the time in simple contexts: "By the time I finish, I will be tired" is correct, but for simple sequence, "when I finish" is more natural.

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Related Articles

GRAMMAR Conjunctions in English: A Complete Guide GRAMMAR English Tenses: A Complete Overview GRAMMAR English Conditionals: Zero, First, Second, Third

Frequently Asked Questions

What is a time clause in English grammar?
A time clause is a subordinate (dependent) clause that tells us when an action happens in relation to another action. It is introduced by a time conjunction such as when, while, before, after, until, as, as soon as, or by the time. A time clause cannot stand alone as a sentence — it must be attached to a main clause to form a complete, meaningful sentence.
Why can't I use 'will' after time conjunctions?
In English grammar, time clauses with a future meaning use the present simple (or present perfect), not will. This is because will is already expressed in the main clause: "I will call you when I arrive" — the "will" in the main clause covers the future time. Using will in both clauses ("I will call you when I will arrive") is grammatically incorrect and sounds unnatural to native speakers.
What is the difference between 'when' and 'while'?
"When" is used for a specific point in time or a short action: "When she arrived, the party started." "While" is used for an ongoing background activity during which something else happens: "While she was talking, I was taking notes." The key difference is duration: when = a point or moment; while = a period or process. Both can sometimes be used interchangeably, but while always implies that the activity was in progress.
When do I use the past perfect in time clauses?
The past perfect (had + past participle) in a time clause emphasises that one past action was fully completed before another past action began. "After she had finished her presentation, she answered questions" makes it clear the presentation was completely over before the questions began. In many contexts, the simple past works fine too: "After she finished, she answered questions." The past perfect adds precision but is not always obligatory.
What is the difference between 'until' and 'by the time'?
"Until" describes a state or action that continues up to a specific point: "I waited until she called." The activity (waiting) was ongoing throughout that period. "By the time" introduces a reference point and is used with the future perfect or past perfect to show that one action will be (or was) completed before that reference point: "By the time she called, I had already left." The key difference is continuous duration (until) vs completion before a point (by the time).
Can 'as soon as' be used in the same way as 'when'?
"As soon as" and "when" can often be used interchangeably for future sequences, but "as soon as" emphasises immediacy and urgency. "Call me when you arrive" is neutral. "Call me as soon as you arrive" means "the moment you arrive — do not wait." In most cases, "as soon as" implies that the second action should follow the first with no delay, making it stronger and more emphatic than "when".
Do I need a comma with time clauses?
Yes, when the time clause comes first in the sentence, a comma separates it from the main clause: "When I get home, I will cook dinner." When the main clause comes first, no comma is needed: "I will cook dinner when I get home." This is the standard punctuation rule for all subordinate clauses in English, not just time clauses. Applying it correctly demonstrates grammatical awareness.
What tense do I use with 'as soon as' for future events?
As with all time conjunctions, use the present simple (or present perfect) — not will — in the as-soon-as clause when referring to the future: "I'll let you know as soon as I hear anything" (present simple). To emphasise that the first action must be fully completed before the second begins, you can use present perfect: "I'll let you know as soon as I have confirmed the details."
Can 'before' be used with the present perfect in a time clause?
Yes: "Make sure you have read the instructions before you start" uses the present perfect (have read) in the before-clause to emphasise that the reading must be fully completed prior to starting. Without the perfect: "Make sure you read the instructions before you start" is also correct and more common in everyday speech. The present perfect version is more precise and slightly more formal.
Is 'by the time' used with future perfect or simple future?
"By the time" is typically used with the future perfect (will have done) in the main clause to show that an action will be completed before the time clause event: "By the time you arrive, I will have finished cooking." The time clause itself uses the present simple ("you arrive", not "you will arrive"). For past sequences, "by the time" pairs with the past perfect in the main clause: "By the time they called, I had already eaten."