while using 'does' we use the form of the verb. The use of does in itself signifies singular so we don't need to use another singular verb. eg. He knows… He doesn't know. She eats… She doesn't eat. While framing the question we say, " does he write well" ? answer- yes he writes well. if we use does, the answer would be " yes he does write well." hope it's clear.

The helping verb "do" takes the he/she/it conjugation: "does". Helping verbs do rather clever things. If you ask "What does it means?", you are using the grammar rule twice when you need to use it only once.

It mean_s_ = It _does_ mean what _does_ it mean?

what's the meaning of ........? the same using but about yours no need to add s/es when the helping verb is found .