Both so that and in order that are used to talk about purpose.
So that is much more common and less formal than in order that. Note that we use auxiliary verbs (can, may, will etc.) in the clause following so that / in order that.
- I am leaving early so that I can catch the morning flight.
- I am putting the meat in the oven, so that it will be ready by the time they return.
In sentences about the past, we use could or would. In British English, should is sometimes used instead of would.
- She spent an extra week in the city so that she could visit all interesting places.
- Doctors removed the tumor in order that cancer wouldn’t spread to other parts.
- I called her so that we could talk.
- His birthday was on Friday but we held the party on Sunday in order that everybody would be free to attend.
Sometimes we use present tenses after so that / in order that to refer to the future.
- Tell her everything so that she knows the truth. OR Tell her everything so that she will know the truth.
- Send the money now itself so that she receives / will receive it by evening.
The same ideas can be expressed using in order to.
- The doctors removed the tumor in order to prevent cancer from spreading to other parts.
- I called her in order to talk. (Very formal)
- She spent an extra week in the city in order to visit all interesting places.
Instead of so that, we can use so as to.
- We are starting early so that we don’t miss the flight. OR We are starting early so as not to miss the flight.
- I had an afternoon nap so that I wouldn’t fall asleep during the meeting.
- I had an afternoon nap in order not to fall asleep during the meeting.
- We wear warm clothes during winter so that / in order that we don’t get cold.
- We wear warm clothes during winter in order not to get cold.
- We wear warm clothes during winter so as not to get cold.
In an informal style, that is often omitted from so that.
- I called her so that we could have a chat. OR I called her so we could have a chat.