We often use an infinitive to talk about a person's purpose — why he or she does something.

• I sat down for a minute to rest
  
• He went abroad to forget
  
• I'm going to Austria to learn German

In a more formal style, we often use in order to or so as to.

• He got up early in order to have time to pack.
• I moved to a new flat so as to be near my work.

In negative sentences, we nearly always use the structure with so as not to or in order not to.

• I'm going to leave now, so as not to be late. (NOT I'm going to leave now, not to be late.)