Reasons

So that vs. to + infinitive: expressing purpose

English has various ways of expressing purpose. We can use so that or in order to; sometimes we can simply use the verb infinitive.

Expressing purpose with a verb infinitive

We use to + infinitive when we want to show the purpose of an action, i.e. to say why we did it:

Tom: Why did you go to London?

Kate: To see my sister.

'To see my sister' = the reason Kate went to London.

We can use to + infinitive when we want to say why something exists:

The information desk is there to help tourists.

The police were there to control the crowd.

We can use to + infinitive after adjectives like easy, hard, difficult, impossible.

It's hard to speak a foreign language fluently.

It's impossible to walk on water.

She finds it easy to make new friends.

So that: expressing purpose

We often use to + infinitive to say why we do something:

I went to the bank to get some money.

('To get some money' is the purpose.)

Sometimes, however, we cannot use the infinitive: we need to use so that.

We use so that to say that one person does something and as a result someone else can do or doesn’t have to do something else:

We built a pool in the garden so that the children can / could swim in hot weather.

He bought his wife a car so that she doesn’t / didn’t have to take the bus so often.

They guarded the prisoner well so that he didn’t / wouldn’t escape.

We can use so that + can / could / will be able to / would have to say that one action makes another action possible:

I’ve bought a dictionary so that I can learn more.

I bought a dictionary so that I could learn more.

I’m going to buy a dictionary so that I can (or will be able to) learn more.

We arrived at the theatre early so that we would have time to eat before the show.

We use so that + don’t have to / won’t have to / wouldn’t have to, to say that one action will help us avoid having to do something else:

I’m going shopping this evening so that I don’t / won’t have to go tomorrow.

I go shopping on Friday evenings so that I don’t have to go at the weekends.

I went shopping on Friday evening so that I didn’t / wouldn't have to go at the weekend.

We use so that + won’t / wouldn’t / don’t to say that one action will prevent another thing happening.

I’ve brought an umbrella so that I don’t / won’t get wet.

I’m going to bring an umbrella so that I don’t / won’t get wet.

I took an umbrella so that I wouldn’t / didn't get wet.

I always carry an umbrella so that I don’t get wet.

So that or so?

In informal speech, native English speakers often say only so instead of so that. However, this can be confusing for learners and we recommend you use so that in its full form.

Formal alternatives to so that:

Instead of so that I could learn, we can say in order to learn.

Instead of so that I didn’t have to learn, we can say in order not to have to learn.

In order to and in order not to have to are more formal than so that.

Before stative verbs (know, have, seem, appear, etc.), we often use in order to or so as to instead of so that.