Declarative: I schedule meetings on Mondays.
Interrogative: Do you schedule meetings on Mondays? Yes, I do.
Declarative: I understand you.
Interrogative: Do you understand me? Yes, I do.
SIMPLE PAST TENSE
Declarative: I scheduled meetings on Mondays.
Interrogative: Did you schedule meetings on Monday? Yes, I did.
Declarative: I understood you.
Interrogative: Did you understand me? Yes, I do.
In English questions, you need an auxiliary verb and a main verb (except for the verb be which works differently).
In the present, the auxiliary for an action verb is DO or DOES [third person].
Do you want some coffee? Does he want coffee?
In the past, the auxiliary is DID.
Did you want some coffee?