Learn Turkish: Sen iyi bir doktorsun. - You are a good doctor.

In Turkish, the verb 'to be' (am/is/are) isn't a separate word. Instead, it's added as an ending to the noun or adjective. The -sun ending specifically means 'you are', so doktorsun means 'you are a doctor'.

Why is the ending -sun instead of -sin?

This is because of 4-way vowel harmony. The ending for 'you are' changes its vowel to match the last vowel of the word it attaches to. Since the last vowel in doktor is 'o', the suffix vowel becomes 'u', giving us -sun.

Why is it iyi bir doktor instead of bir iyi doktor?

In English, we say 'a good doctor', putting 'a' first. In Turkish, when you use an adjective (like iyi) with the word bir (a/an), bir almost always goes between the adjective and the noun.

Is the word Sen (you) mandatory in this sentence?

No, you can leave it out and just say İyi bir doktorsun. Because the ending -sun already means 'you are', the pronoun Sen is optional and is usually just kept for emphasis.