"This is Professor Mohamed's room."

Translation: هٰذِهِ غُرْفة اَلْأُسْتاذ مُحَمَّد.

[NorahAlotaibi]

I keep writing this correctly (using an Arabic keyboard) but it keeps marking it wrong

[MasoudTaghavi]

Is it هذه instead of هذا because room is female?

[tzuriaf]

Yes, "this" is referring to the room, and the room is feminine, so "this" must be in the feminine form.