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- The zero conditional uses if + present simple, present simple.
- It describes things that are always true — facts, scientific laws and general truths.
- If can usually be replaced by when with the same meaning.
- It is also used for instructions and automatic results.
- Both clauses stay in the present simple — there is no will.
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Of all the English conditionals, the zero conditional is the easiest to learn and the easiest to misuse. It describes situations where one thing always leads to another — cause and effect that is true every single time. Because the result is certain, both halves of the sentence stay in the present simple. This guide explains how to build the zero conditional, exactly when to use it, how it differs from the first conditional, and why if and when are often interchangeable here.
How to Form the Zero Conditional
The structure is symmetrical: the if-clause and the main clause both use the present simple.
The Structure
| If-clause (condition) | Main clause (result) |
|---|---|
| If you heat water to 100°C, | it boils. |
| If you mix blue and yellow, | you get green. |
| If I drink coffee at night, | I don't sleep well. |
The order can be reversed without changing the meaning: "Water boils if you heat it to 100°C." When the if-clause comes first, a comma separates the two clauses; when it comes second, no comma is needed.
Facts and Scientific Truths
The classic use is for things that are always true — laws of nature and general facts.
If you don't water plants, they die.
Ice melts if you leave it in the sun.
If you press this button, the machine starts.
Habits and Routines
It also describes personal habits — results that follow a condition every time for a particular person.
If I'm tired, I go to bed early.
She gets a headache if she reads in the car.
Instructions and Commands
When giving instructions, the result clause often uses an imperative instead of a present-simple statement.
If the light turns red, stop.
If you have any questions, ask the teacher.
If it doesn't work, restart the computer.
If vs When
Because the result is always true, if and when mean almost the same thing in the zero conditional.
If you heat ice, it melts. = When you heat ice, it melts.
Both are correct. When can slightly stress that the situation regularly happens, while if presents it as a general rule, but the difference is small.
Zero vs First Conditional
Key Difference
| Zero conditional | First conditional |
|---|---|
| Always-true result | Likely future result |
| If you heat water, it boils. | If it rains, I will stay home. |
| present + present | present + will |
The zero conditional describes general truths; the first conditional describes a specific future possibility. The grammatical signal is the verb in the result clause: present simple for zero, will for first.
Common Mistakes
The most common error is using will in the result clause ("If you heat water, it will boil") — correct for a prediction, but for a general fact the present simple is standard. Another mistake is adding will after if ("if you will heat"), which is wrong in all basic conditionals. A third is forgetting the third-person -s ("if she read" should be "if she reads"). Keeping both clauses in the present simple solves nearly every zero-conditional problem.
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