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MULTIMEDIA-ENGLISH grammar sheet |
How to express purpose – Level: 
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- Afirmative
- Negative
- Different subjects
- General purpose (definitions)
https://multimedia-english.com/grammar/how-to-express-purpose-41
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)