Infinitive of purpose: I sat down to rest.

I sat down to rest.

We often use an infinitive to talk about a person’s purpose – why he or she does something.

• I sat down to rest. (not I sat down for resting / for to rest.)
• He went abroad to forget.
• I’m going to Austria to learn German.
• To switch on, press red button.

in order to; so as to

We can also use in order to … (more formal) or so as to …

• He got up early in order to have time to pack.
• I watched him in order to know more about him.
• I moved to a new flat so as to be near my work.

In order to / so as to are normal before negative infinitives.

• I’m going to leave now, so as not to be late. More natural than I’m going to leave now, not to be late.

A for-structure (see here) can be used to talk about a purpose that involves action by somebody else.

• I left the door unlocked for Harriet to get in.