"Einer deiner Lehrer" – is this sentence correct?

Question

Okay so I am reading through Harry Potter in German and I have come across this sentence:

Harry, Professor Quirrell ist einer deiner Lehrer in Hogwarts.

I do not understand why it is "einer deiner Lehrer" when "deiner" should be demonstrative and "einer" is indefinite pronoun.

Should it not be "einer dein Lehrer"?

Answer 1

"deiner Lehrer" is the genitive plural of "dein Lehrer". The construction "einer/eine/eines + genitive plural" corresponds to the English "one of ...", so it's "one of your teachers".

Answer 2

Think of that as if the following steps were applied:

1 - ein Lehrer von deinen Lehrern

Written like that it's a bit ambiguous, but bear with me. Read it as "one of your teachers", the more logical meaning, rather than "a teacher of your teachers".

Now let's replace the "ein Lehrer" with the corresponding pronoun:

2 - einer von deinen Lehrern

Finally, let's replace "von + dative" with the genitive:

3 - einer deiner Lehrer

Hope this makes it clear.