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So that and in order that, where a clause rather than an infinitive expresses purpose.
Why you specifically
The rule
Purpose clauses use so that and in order that when the purpose is expressed with a full clause containing a subject and a finite verb. This is different from the infinitive of purpose, such as to catch the bus or to find a seat, where no subject is stated in the purpose phrase. So that is common in speech and writing; in order that is more formal and less frequent. After these connectors, the verb is often in a modal clause such as could, would, or can, depending on time and meaning.
Both correct
Every sentence here is correct English. Only the meaning changes.
I left early so that I could catch the last bus.
Purpose with a clause: the action is done to achieve a result.
I left early to catch the last bus.
Purpose with an infinitive: the action is done to achieve a result, but the purpose phrase uses to + verb.
We turned off the lights so that the baby could sleep.
Purpose with a clause: the speaker explains why the lights were turned off.
We turned off the lights to let the baby sleep.
Purpose with an infinitive: the speaker explains why the lights were turned off, using to + verb.
The trap
I left early so that catch the last bus.
I left early so that I could catch the last bus.
Use so that + clause when you need a subject and a verb after the purpose connector.
She spoke slowly in order that everyone understand her.
She spoke slowly in order that everyone could understand her.
In order that is followed by a clause, not by a bare verb form.
We turned off the lights so that the baby sleep.
We turned off the lights so that the baby could sleep.
A purpose clause needs a finite verb such as could sleep; a bare infinitive does not follow so that.
Prove it
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