"Ο δάσκαλος μου είχε δώσει την απάντηση."

There is no need for punctuation. "My teacher..." would have to be "Ο δάσκαλός μου...", i.e. with a double accent on 'δάσκαλος' (the reason being that the possessive pronoun is considered part of the preceeding noun and every Greek multi-syllable word needs an accent on one of the last three syllables). So there is no ambiguity and the translation is clear.

In cases where the accent of the noun is not on the antepenultimate syllable as in δάσκαλος, but rather on the penultimate or last syllable the situation is more complicated. If we use 'άντρας', for example, and want to say "The man told me..." then we do not use punctuation, but rather have to put an accent mark on the personal pronoun: "Ο άντρας μού είπε..." - thus avoiding confusion with "Ο άντρας μου είπε..." ("My husband said...). Please note that this accent on the personal pronoun is only used in cases of ambiguity.