'Had' is past tense of 'have' (to own, to possess) and 'had to' is past tense of 'have to' (to be obliged to*) Examples: I had a pencil in my pocket yesterday. (I owned/possessed a pencil that was in my pocket) I had to do many tasks for homework two days ago. (I was obliged* to do many tasks...) * "to be obliged to" is not really a synonym of the verb/phrase "to have to" but rather a close approximation. Nevertheless, I hope you understood the example I gave.

"Have" is a very flexible verb in English, so a short answer won't cover all possibilities. However, "have to" (past tense is "had to") can mean an obligation. "At school, I had to wear a uniform."