1.He does not want to tell a lie. he is a singular subject but we used want instead of wants why ? 2.Does he write a letter? Here he is also singular and we used write instead of writes please explain
"Want" is not (here) a plural verb: it is the base form (sometimes called the infinitive), because the auxiliary "do/does/did" always always always takes the base form of the following verb. The same for "write".
"Does" is the verb that is (3rd person) singular, to agree with "he".