Syntax

Purpose clauses

We use purpose clauses to indicate the purpose of an action.

He left early to catch the train.

There are two kinds of purpose clauses: non-finite clauses and finite clauses.

Non-finite purpose clauses are the most common and contain to + infinitive. Common conjunctions used in non-finite purpose clauses are: to, in order to and so as to.

He left early to catch the train.
To inflate the life jacket, pull the cord.
He left early so as to catch the train.
He left early in order to catch the train. (more formal)

The subject of a non-finite clause is the same as the subject of the main clause. To make the sentences negative we put not in front of the to.

She woke up early in order not to miss the train.
She woke up early so as not to miss the train.

But we can't negate a sentence that contains only to + infinitive.

She woke up early not to miss the train.
They built a fence not to let the dogs in.

We can, however, express these ideas by:

- changing the sentence.

She woke up early to catch the train.
They built a fence to keep the dogs out.

-using a finite clause.

She woke up early so that she wouldn't miss the train.
They built a fence so that the dogs couldn't get in.

Finite purpose clauses usually contain that + clause and they usually contain a modal.

They took a torch so that they could see when it got dark.

Common conjunctions used in finite purpose clauses are so, so that and in order that.

Write down the number so you don't forget it.
I took an umbrella so that I wouldn't get wet.
Write this down in order that there may be no misunderstanding. (more formal)

If the verb in the main clause is in the present or present perfect tense, we usually use the modals can, may, will or shall in the purpose clause.

I'm studying English so that I can use the Internet.
She explains it thoroughly in order that you may understand.
They are working overtime so that they will earn more money.
We commemorate this day in order that we shall not forget this historic moment.

If the verb in the main clause is in the past tense, we usually use could, might, or would in the purpose clause.

I got up early so that I could be ready for the meeting.
I stood on a chair so that I might see the procession better.
They took a lot of photos in order that they would have a record of the important day.

We sometimes use ordinary verbs in the purpose clause, usually in negative purpose clauses.

I remind him so that he doesn't forget.
I'm wearing a hat so that I don't get sunburnt.

In negative purpose clauses in case can also be used.

I remind him in case he forgets.
They took an umbrella in case it rained.
She drew a map in case she couldn't find her way home.