AFFIRMATIVE   to + infinitive

- He started drinking to forget
- I need a chair to sit down
 
NEGATIVE   so as not to + infinitive

- I'll take a taxi so as not to be late
- Take your shoes off so as not to wake them up
 
DIFFERENT SUBJECTS    so that + subject + can

- I'll open the door so that he can enter
- She covered the hole so that water couldn't come in
 
NOTE: we use could when the main clause is in the past. instead of can/could you may also find: may/might, will/would/should
 
More colloquial  for + subject + to

(+)  I'll open the door for you to enter
(-)   I'll close the door for you not to go out

GENERAL PURPOSE (DEFINITIONS)   for + -ing

      A tin opener is something for opening tins
      That's for peeling potatoes    

(remember that after prepositions and conjunctions we use -ing)
 
but if we are talking of a particular use of something we can't use "for"

      I need a tin-opener to open this tin

You can never say FOR TO !

She went home to have dinner  (not: She went home for to have dinner)