In English sentences, a lot of important meanings are expressed by the form of the verb phrase — for example questioning (this is what your question is about), negation, time, completion, continuation etc. But English verbs do not have many different forms. So to express all these meanings, a number of “auxiliary” (or “helping”) verbs are added to other verbs.
1. BE
Be is added to other verbs to make continuous and passive forms. WAS is the past tense of IS and AM, WERE is the past tense of ARE. They are all forms of BE.
IS it raining?
Was Peter arrested last night?
2. DO
DO is used to make questions (and negative) forms of non-auxiliary verbs, i.e. main verbs. DID is the past tense of DO and DOES.
DO you smoke?
DID he go to the party last Sunday?
3. HAVE
HAVE is used to make perfect forms. HAD is the past tense of HAVE and HAS.
HAVE you finished your homework?
HAS he talked to you?
4. MODALS
Modal verbs WILL, WOULD, SHOULD, CAN, COULD, MAY, MIGHT, MUST etc. are also called “modal auxiliary verbs”. They are used with other verbs to add various meanings, mostly to do with degrees of certainty or obligation.
CAN he swim?
WILL you call me tomorrow?
Modals can be auxiliaries:
She can play tennis. Can Sam play football? Will you call me tomorrow? The computer will not start up!
DO/DOES are the auxiliaries used to ask questions in the present (not with the Verb BE):
DO they play tennis every day? DOES she know Jack? DOES Sammy have a brother?
DID is used to ask questions in the past (with these same Verbs):
Did they play tennis yesterday?
Did you call me?
Did they arrive in time to catch the train?
Also in negatives:
I do not understand.
I did not study German.
He did not phone me last night.
In short answers:
Do you have a brother? – Yes, I do.
Does Ellen work for Mr. Jackson? – Yes, she does.
Did you understand the last lesson? – Yes, I did.
WAS is the past tense of AM and IS; WERE is the past tense of ARE. They are all forms of the Verb BE:
I am a shipping clerk.
How old is Sam? – He is 28 years old.
I was at home this morning.
Peter was not very interested in this subject.
Was John from Germany?
Remember that the Verb BE can also be used as an auxiliary to form continuous tenses and Passive Verbs:
Ellen IS WASHING the dishes. (Present Continuous)
The dishes ARE BEING WASHED. (Present Continuous Passive)
Practise grammar along with vocabulary you are learning and it will become natural to you. Find five minutes to speak English every day and your fluency will improve!