'to think ahead' means to make plans or arrangements for the future.
- Just concentrate on today and try not to think too far ahead.
- We're already thinking ahead to what will happen after the elections.
'to think back' means to look back over things that have happened.
- When I think back on what I did, I feel embarrassed.
- Think back to when I first lived here and what the house was like then.
'to think of' a fact or something that exists means you know it and can suggest it to someone else.
- I can't think of any examples of something he has done well.
- Can you think of any reason I should do it? I'm not going to get paid.
'to think of' also means to consider the possibility of doing something.
- I'm thinking of going to Portugal for the holidays.
- He's thinking of starting his own business.
'to think out' means to prepare or plan something fully before you start doing it.
- He obviously hadn't thought it out properly before starting.
- It was very well thought out. He had obviously spent a lot of time on it.
'to think over' means to consider something carefully before deciding.
- We've got all the information we need. We'd like to think it over.
- I just needed a bit of time to think it over before I told him what I had decided.
'to think through' means to consider something carefully with all the possible consequences.
- It sounds like a good idea but we need to think it through to see if it will work.
- I haven't had time to think it through at all. I don’t know what will happen.
'to think up' means to create something using your imagination.
- I don't want to go to their party but I can't think up an excuse.
- We'll have to think up a very good reason why we didn't make the deadline.