Look at these examples:
- I phoned the restaurant to reserve a table.
- What do you need to make bread.
- We shouted to warn everybody of the danger.
- This letter is to confirm the decisions we made at our meeting last week.
- The president has a team of bodyguards to protect him.
In these examples the word 'to' tells us the purpose of something: why somebody does something, has something, needs something etc., or why something exists.
You can say 'for somebody to do something':
- There weren't any chairs for us to sit on, so we sat on the floor.
You can use for -ing or to... to talk about the general purpose of something, or what it is generally used for:
- I use this brush for washing the dishes. or ...to wash the dishes.
We use so that especially:
When the purpose is negative
- I hurried so that I wouldn't be late.
with can and could
- She's learning English so that she can study in Canada.
You can leave out that. So you can say:
- I hurried so that I wouldn't be late. or I hurried so I wouldn't be late.