Learn Turkish: Yani sen doktor değilsin.

Yani translates to "so," "I mean," or "in other words." It is a very common connecting word in Turkish used to draw a conclusion or clarify something, just like how we use "so" in English.

Why is there an ending on değilsin instead of just using değil?

In Turkish, to say "am not," "is not," or "are not" with a noun, the negative word değil (not) takes the personal ending. Since the subject is sen (you), we add the "you" ending -sin directly to değil, making it değilsin (you are not).

If değilsin already means "you are not," do we really need the word sen here?

No, you don't strictly need it. Because the -sin ending on değilsin already shows who is being talked about, the word sen (you) could be dropped. Including it in this sentence just adds a little extra emphasis to "you."

Why isn't there a word for "a" before doktor?

Turkish generally drops the word bir (a/an) when stating someone's profession or identity. While in English you say "you are not a doctor," in Turkish it is perfectly natural to just say "you are not doctor."