1. В том, что мы не ставим перед ним вс глагол don't/doesn't, а используем отрицательную форму глагола (с частицей not).
I am not a doctor [не I don't be a doctor/I don't am a doctor]
We are not doctors [не We don't be a doctor/We don't are a doctor]
He is not a doctor [не She doesn't be a doctor/She doesn't is a doctor]
2. Так же - мы не ставим вс глагол do/does на начало предложения, но ставим одну из форм глагола to be (в зависимости от лица).
Am I a doctor? [не Do I be a doctor?/Do I am a doctor?]
Are we doctors? [не Do we be doctors?/Do we are doctors?]
Is he a doctor? [не Does he be a doctor?/Does he is a doctor?]
3. always (всегда), often (часто), every day (каждый день), every year (каждый год), every week (каждую неделю), usually (обычно), sometimes (иногда), seldom (редко) (подробный список можно посмотреть в Интернете)
2. as
3. as
4. than
5. as
6. than
7. as