We use certain clauses to show why somebody does something. These are called clauses of purpose. They are introduced with the following words or expressions:to + infinitive; David went out to buy a bottle of wine.
in order to/so as to + infinitive (formal); We were asked to say over in order to finish the project.
so that + can/will – used for a present or future reference; Here’s my number so that you can call me if you have a problem.
so that +could/would – used for a past reference; We left early so that we would be able to park close to the stadium.
in case + present tense – present or future reference – Take your hat in case it gets too hot.
in case + past tense – used for past reference – We took an umbrella in case it rained.
for + noun – used to express purpose – We went out for a pizza.
for + -ing – used to express purpose and/or function of something – This function on the air conditioner is for reducing humidity.
NegativesFor negative purpose we use:
in order not to/so as not to + infinitive – we walked in quietly so as not to wake up the children.
prevent + noun/pronoun + from + ing – I parked the car under a tree to prevent it from getting too hot.