English — Purpose clauses: so that / in order that
To say why someone does something, English uses a purpose clause with so that (or, more formally, in order that) followed by a full clause, usually with can, could or would: I left early so that I could catch the train.
How it works
Use so that / in order that + a clause when the purpose has its own subject and verb, typically a modal: She spoke slowly so that everyone could understand. The present-tense verb follows a present main clause (I'm writing this down so that I don't forget); a past modal follows a past main clause (I turned it off so that it wouldn't ring).
Clause vs to-infinitive
| Structure | Example | When to use it |
|---|---|---|
| to + infinitive | I went to the shop to buy milk. | Same subject in both parts; the shortest, most common option. |
| so that + can/present | I'll email you the file so that you can print it. | Different subject, or a modal is needed; neutral register. |
| so that + could/would (past) | She whispered so that the baby wouldn't wake up. | Purpose in a past context; the modal shifts to past. |
| in order that + clause | The rules were changed in order that all staff might benefit. | Formal or written; less common than so that. |
| in order to + infinitive | We arrived early in order to get good seats. | More formal equivalent of the to-infinitive; same subject. |
Good to know
A to-infinitive can have an understood subject supplied by an object: I gave him a key to open the door is grammatical and normally means that he will open it. Use so that he could open the door when you want the subject to be fully explicit. Also avoid inserting for after so that (not so that for you can see); the that itself is often dropped in speech: I moved my car so you could park.