A purpose clause answers the question ‘why?’ by stating the goal or intention behind the action in the main clause. English offers several ways to express purpose, ranging from the simple infinitive to more formal structures used in writing. Reference grammars such as the Cambridge Grammar group these under clauses of purpose, distinct from clauses of reason or result. For example, She left early in order to catch the train and the more formal She left early so that she could catch the train both express the same purpose. Learners often drop the necessary to, writing She left early for catch the train, which is incorrect.
What Is a Purpose Clause?
A purpose clause is a subordinate clause (or reduced phrase) that tells you the intended outcome of an action. It typically follows the main clause and answers ‘in order to do what?’
- She saves money to buy a house. (infinitive of purpose)
- He left early in order to catch the train. (more formal)
- They whispered so that no one would hear. (clause with subject + modal)
Formation Table
| Structure | Example | Register |
|---|---|---|
| to + infinitive | I called to check the time. | Neutral, most common |
| in order to + infinitive | I called in order to check the time. | More formal / written |
| so as to + infinitive | I called so as to check the time. | Formal / written |
| so that + subject + will/can/would/could | I called so that I could check the time. | Neutral, needs a different subject or a modal |
| for + noun / -ing (fixed use) | This tool is for cutting wood. | Describes general purpose of an object |
Negative Purpose
To express a purpose you want to avoid, use so as not to, in order not to, or so that…not/wouldn’t. A bare infinitive cannot be negated this way with ‘not to’ alone in careful writing without one of these connectors.
- She left early so as not to miss the bus.
- He whispered in order not to wake the baby.
- They locked the door so that no one could get in.
When to Use ‘So That’ Instead of an Infinitive
Use so that (with a full clause containing a modal verb) when the subject of the purpose clause is different from the subject of the main clause, or when you need a modal such as can, could, will, would to show ability or possibility.
- I’ll give you a map so that you can find the hotel. (different subjects: I / you)
- She spoke slowly so that everyone could understand. (different subjects: she / everyone)
Common Mistakes
- Wrong: I went to the shop for to buy milk. Correct: I went to the shop to buy milk. (‘for to’ is never standard English)
- Wrong: She studies hard so that pass the exam. Correct: She studies hard so that she can pass the exam. (‘so that’ needs a subject and usually a modal)
- Wrong: He ran fast for not to be late. Correct: He ran fast so as not to be late. (the negative infinitive of purpose needs ‘so as not to’ or ‘in order not to’)
- Wrong: This knife is for cut bread. Correct: This knife is for cutting bread. (‘for’ + purpose takes the gerund, not the infinitive)
Practice Exercises
Grammar Quiz
Choose the correct purpose structure for each sentence.
Cloze Dropdown
Select to, in order to, so as to, or so that in context.
Complete the Sentence
Add a purpose clause that logically fits each sentence stem.
Match-Up
Match actions to the purpose clauses that explain them.
Flash Cards
Practise forming purpose clauses with instant feedback.
Practice What You’ve Learned
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Frequently Asked Questions
What is a purpose clause?
A purpose clause explains the goal or intention behind an action — it answers the question ‘why?’ in terms of what someone hoped to achieve. Common structures include the infinitive of purpose (to buy milk), in order to, so as to, and so that.
What is the difference between ‘to’ and ‘in order to’?
Both express purpose and are usually interchangeable: I called to check / I called in order to check. ‘In order to’ is more formal and more common in writing, while the simple infinitive ‘to’ is the everyday, neutral choice in speech and casual writing.
When should I use ‘so that’ instead of an infinitive?
Use ‘so that’ with a full clause when the subject of the purpose differs from the subject of the main clause (I’ll give you a map so that you can find the hotel), or when you need a modal verb such as can, could, will, or would to express ability, permission, or future possibility.
How do I express a negative purpose?
Use so as not to, in order not to, or so that…not/wouldn’t. You cannot simply say ‘not to’ on its own for this meaning in careful English: She left early so as not to miss the bus (not ‘for not to miss’).
Is ‘for to buy milk’ correct?
No. ‘For to’ before an infinitive is never standard English, though it appears in some non-standard dialects and is a common learner error. The correct form is simply the infinitive of purpose: I went to the shop to buy milk.
What is the difference between ‘for + noun’ and ‘to + verb’ when expressing purpose?
‘For’ is used with a noun or gerund to describe the general purpose of a thing: This knife is for cutting bread. ‘To + infinitive’ is used to describe the purpose of a specific action performed by a person: I used this knife to cut the bread. Mixing them up (‘for cut bread’, ‘to cutting bread’) is a common error.
Can a purpose clause come at the start of a sentence?
Yes, for emphasis or stylistic variety: To catch the early train, she woke up at five. In order to succeed, you must practise daily. When a purpose clause opens the sentence, it is usually followed by a comma before the main clause.
What is the difference between a purpose clause and a reason clause?
A purpose clause states an intended goal (I studied hard to pass the exam — the goal was passing). A reason clause states a cause that already exists (I studied hard because the exam was difficult — the difficulty is the existing cause). Purpose looks forward to a hoped-for outcome; reason explains an existing fact.
Is ‘so as to’ common in spoken English?
‘So as to’ is more common in formal written English — reports, academic writing, instructions — than in everyday conversation, where the simple infinitive of purpose (‘to’) is far more natural. Overusing ‘so as to’ in casual speech can sound stiff.
Why are purpose clauses tested in exam writing tasks?
Purpose clauses allow writers to link an action clearly to its intended goal in a single sentence, which is valued in IELTS and Cambridge exam writing for demonstrating cohesion and grammatical range. Using a variety of purpose structures — not just repeating ‘to’ every time — is rewarded under lexical and grammatical range criteria.