I would like to know if this sentence is correct:
optimus magister is fuit qui scivit quid discipuli peterent: auxilium
What I would like to say in English is: "The best teacher was the one who knew what students were looking for: help".
I don't know what the colon in Latin was. Thus I used the English punctuation.
Scivit should be subjunctive sciret, since (on the most natural reading) this is a relative clause of characteristic: whoever knew what students were looking for, that person was the best teacher.
I'm doubtful about the colon with auxilium. My sense is that Latin prose doesn't generally use this construction in the way that English does, but would be more likely to use something like a "namely" instead. You could use id est for that.
You might consider changing fuit to imperfect erat, but that depends on how you want to present the aspect of this sentence and on the context.
Is could optionally be left out -- grammatically the sentence is fine either way, but the rhythm may be a bit better without it.
Finally, I would change the order of the indirect question clause to quid peterent discipuli, which to me sounds better.
So: Optimus magister (is) fuit/erat qui sciret quid peterent discipuli, id est auxilium.
