Complete the sentences using can / can't / could / couldn't.

1. I’m doing well at school. I got top marks for my English essay. I think it’s because I write really well.

2. I got a bad mark in my exams. My problem is that l remember facts.

3. I was nervous last year in my science exam that I remember a thing.

4. I did really badly in my history exam last summer. I didn't do enough homework so I understand the subject very well.