think
The verb think is used in several different ways. Its past tense and -ed participle is thought.
used with a that-clause
You can use think with a that-clause when you are giving your opinion about something or mentioning a decision that you have made.
I think you should go.
I thought I'd wait.
When you use think like this, you usually use a simple form, but in conversation you can use a progressive form, especially if you want to emphasize that your opinion or decision might change.
I have too many books. I'm thinking I might sell some of them.
Instead of saying that you think something is not the case, you usually say that you don't think it is the case.
I don't think this will work.
I don't think there is any doubt about that.
‘I think so’
If someone asks you whether something is the case, you can express your opinion that it is probably the case by saying ‘I think so’. Don't say ‘I think it’.
‘Do you think my mother will be all right?’ – ‘I think so.’
If you want to reply that something is probably not the case, you usually say ‘I don’t think so'.
‘I have another friend, Barbara Robson. Do you know her?’ – ‘I don’t think so.'
‘Are you going to be sick?’ – ‘I don’t think so.'
using a progressive form
When someone is thinking, they are considering something. When you use think with this meaning, you often use a progressive form.
I'll fix us both a sandwich while I'm thinking.
You have been thinking, haven't you?
You also use a progressive form when you are talking about what is in someone's mind at a particular time.
That's what I was thinking.
It's very difficult to guess what the other people are thinking.
You can say that someone is thinking about something or someone, or is thinking of something or someone.
I spent hours thinking about the letter.
She was thinking of her husband.
If you are considering doing something, you can say that you are thinking of doing it.
I was thinking of leaving home.
Be careful
Don't say ‘I was thinking to leave home’.
Don't say ‘I was thinking to leave home’.