How is this incorrect?

Question

I was talking to my Italian Professora and we were joking about my attempts to flirt with a person. However, when I told her about the line Tu non sei il sole ma fate la mia mattina (You are not the sun, but you make up my morning); she said my Italian grammar was incorrect. Before she could offer an explanation, she had to charge off to a meeting. My Italian friend does not know why its wrong, and I can't tell either. Should fate be tu fai? Is that my issue?

Answer 1

I was talking to my Italian Professora...

First of all I should warn you that the most used feminine form of "professore" is professor-essa; "professor-a" is not incorrect, but less usual.

As for your sentence,

Tu non sei il sole ma fate la mia mattina,

from a grammatical point of view the problem is what you've understood: you begin the sentence using tu... sei (singular), but conclude it using (voi) fate (plural), as if you're referring not to the same person but to several people or also to the same person but with a less informal tone. Unfortunately both pronouns (tu, voi) are translated by the English word "you".

Moreover, I think you're attempting a literal translation of an idiomatic sentence (or are you referring to Elvis' song?) by saying «fate(/fai) la mia mattina»: if you're referring to the action of the Sun and to the fact that the person you're talking to warms your heart and makes you happy, note that we usually say:

illumini il mio giorno

or

illumini il mio cielo.