Use to , so as to, and in order to to express purpose in the affirmative form.
- He is looking for a part time job to save some pocket money.
- She wakes up early in order to be on time to work.
- They visited him so as to offer their condolences for the death of his wife.
Use so as not to and in order not to to express purpose in the negative form.
- They woke up early in order not to be late.
- She exercises regularly so as not to get fat.
- He helped the new policewoman so as not to fail in her first mission.
You can also express purpose with so that. In this case you generally need to use a modal.
Exercise on expressions of purpose
- She told him to play quiet so that he wouldn't disturb the neighbors.
- He got a visa so that he can travel to the USA.
- He decided to stay in England for a while so that he could practice his English.