a) "had to + infinitive" is for past obligations. b) "must have + past participle" refers to logical deductions about the past e.g. I can't find my cup! Where did I leave it? Oh... I remember now, I was drinking tea while making the lunch. I must have left it in the kitchen - I'll go and check there.

Must have bought a noteboook is not correct. Had to buy a notebook is better. A better exampe would be; Before I started school, I had to buy a book.

Thanks a lot Gary!

Following on from Michael's answer, if you are logically deducing from 'must have bought' - you have a situation where you are deducing from the fact that there is a notebook there, that you must have bought it (and didn't remember buying it!). That's not incorrect, but it is very very unlikely. The second version is more likely, but I think that most people would say 'Before I could take the bus to school, I had to buy a new notebook', as going to school is conditional on getting the notebook.