Main points

* Purpose clauses are introduced by conjunctions such as `so’, `so as to’, `so that’, `in order to’ or `in order that’.

* Reason clauses are introduced by conjunctions such as `as’, `because’, or `in case’.

* A purpose or reason clause needs a main clause to make a complete sentence.

* A purpose clause usually comes after a main clause. A reason clause can come before or after a main clause.

1 You use a purpose clause when you are saying what someone’s intention is when they do something. The most common type of purpose clause is a `to’-infinitive clause.
The children sleep together to keep warm.
They locked the door to stop us from getting in.

Instead of using an ordinary `to’-infinitive, you often use `in order to’ or `so as to’ with an infinitive.
He was giving up his job in order to stay at home.
I keep the window open, so as to let fresh air in.

To make a purpose clause negative, you have to use `in order not to’ or `so as not to’ with an infinitive.
I would have to give myself something to do in order not to be bored.
They went on foot, so as not to be heard.

Another way of making purpose clauses negative is by using `to avoid’ with an `-ing’ form or a noun group.
I had to turn away to avoid letting him see my smile.
They drove through town to avoid the motorway.

2 Another type of purpose clause begins with `in order that’, `so’, or `so that’. These clauses usually contain a modal.

When the main clause refers to the present, you usually use `can’, `may’, `will’, or `shall’ in the purpose clause.
Any holes should be fenced so that people can’t fall down them.
I have drawn a diagram so that my explanation will be clearer.

When the main clause refers to the past, you usually use `could’, `might’, `should’, or `would’ in the purpose clause.
She said she wanted tea ready at six so she could be out by eight.
Someone lifted Philip onto his shoulder so that he might see the procession.

You use `in order that’, `so’, and `so that’, when the subject of the purpose clause is different from the subject of the main clause. For example, you say `I’ve underlined it so that it will be easier.’ You do not say `I’ve underlined it to be easier’.

3 You can also talk about the purpose of an action by using a prepositional phrase introduced by `for’.
She went out for a run.
They said they did it for fun.
I usually check, just for safety’s sake.

4 You use a reason clause when you want to explain why someone does something or why it happens. When you are simply giving the reason for something, you use `because’, `since’, or `as’.
I couldn’t see Helen’s expression, because her head was turned.
Since it was Saturday, he stayed in bed.
As he had been up since 4 am, he was very tired.
You can also use `why’ and a reported question to talk about the reason for an action. See Unit 75.

I asked him why he had come.

5 When you are talking about a possible situation which explains the reason why someone does something, you use `in case’ or `just in case’ .
I’ve got the key in case we want to go inside.
I am here just in case anything unusual happens.

WARNING: You do not use a future tense after `in case’. You do not say `I’ll stay behind in case she’ll arrive later’.