We can express the infinitive of purpose by:
To+infintive: I gave English lessons to earn some extra money.In order to+infintive: I gave English lessons in order to earn some extra money.So as to+infinitive: The Athletics Federation has introduced stricter regulations so as to prevent cheating. Here are other forms where either a noun or verb+ing are used:For+ noun: We used the basement for storage.For+v+ing: The red button is for turning the machine off. The negative is also used: in order not to+infinitive/so as not+infinitive:Examples: *In Tehran the wives of foreign diplomats wore headscarves so as not to offend the Iranians. p. 1416 Mac Millan English Dictionary.*We walked in quietly in order not to wake up the children.
Clauses of purpose: 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.Purpose clauses can also be formed by so that/in order that+ “may”/“shall” in the present and “might”/“should” in the past.Note that so that can be followed by will/can/may/shall or the past forms, while in order that is limited to may/shall or the past forms.
Examples: *These men risk their lives so that/in order that we may live more safely.*We carved their names on the stone so that/in order that future generations should/might know what they had done.
Remember that most of these expressions are used to answer the question “why?” In what concerns for+noun or -ing form the question is “What for?” which means “why?”Mr. ZEGGAI MIMOUN