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
Don't use a to-infinitive when the two subjects differ: not I gave him a key to open the door if he, not you, opens it — say I gave him a key so that he could open the door. 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.