Do + base form in questions
Incorrect : You understand what I say?
Correct : Do you understand what I say?
Incorrect : He loves you?
Correct : Does he love you?
Incorrect: He went there?
Correct : Did he go there?
Note: The auxiliary verb 'do' (and its two other forms "does' and "did') is placed before the principal verb to form questions in the present and past simple tenses.
