how to express... purpose!

Grammar Tables: 

for, to, so that

To talk about purpose, 

or the reason why we do things, 

we can use  for + verb-ing, 

we can use the infinitive with to, 

and we can use so that + subject + verb.

Action or event	'for' + verb-ing
I'm bringing some pots and pans	for cooking.
I've got special glasses	for reading small print.
A thermos flask	is for keeping hot food hot.

Action or event	Infinitive with 'to'
We've got some water	to make tea.
I went to the shops	to buy some milk.
I worked very hard	to get a promotion.

 

Action or event	'so that' + subject + verb
I'm taking some extra blankets	so that we don't get cold.
I chained my bicycle to the fence	so that it wouldn't get stolen.
I worked very hard	so that I would do well in the exam.