Purpose Clauses
Purpose clauses explain why someone does something — the aim or intention behind an action. They answer the question 'What for?' or 'Why?'
Ways to express purpose
1. To + infinitive (most common)
She went to the library to study. (Why? To study.)
He called to apologise.
They moved to London to find better work.
2. In order to / so as to (more formal)
These are more emphatic and formal alternatives to a simple 'to'.
She practised every day in order to improve her score.
He spoke quietly so as not to wake the baby.
They invested heavily in order to expand the business.
3. So that + clause (especially when subjects differ)
Use so that when the purpose involves a different subject, or when a modal verb is needed.
She left early so that she would catch the train.
He spoke clearly so that everyone could understand.
I wrote it down so that I wouldn't forget.
4. For + noun phrase (purpose of an object or place)
This tool is used for cutting wood.
She went to the pharmacy for some medicine.
Negative purpose
To avoid something — use so as not to or in order not to or so that … not.
He whispered so as not to disturb anyone.
She left early in order not to be late.
Write it down so that you don't forget.
Common error: Don't use 'for' + infinitive to express purpose: She went to the shop for buy milk ✗ → She went to the shop to buy milk ✓.
Tip: If both clauses have the same subject, use 'to' or 'in order to'. If the subjects are different, or you need a modal verb in the purpose clause, use 'so that'.