× He was sent to prison, so that he didn’t rob any more banks.
√ He was sent to prison, so he didn’t rob any more banks.
We use so that to describe the purpose of something, the reason why we do something. Don’t use a comma before so that.
We use so to describe the result of something. Use a comma before so.
Compare the following sentences:
- They took a mobile phone so that they could call in case of emergency. (They took the phone to call in case of emergency; that’s the reason why they took the phone).
- They didn’t have a phone, so they couldn’t call the police. (Not calling the police is the result of not having a phone. As they didn’t have a phone, they couldn’t make the call).