Used to, Do, and Does

Used to
• It is used to say that something happened continuously or frequently during a period in the past:-
(1) I used to live in London.
(2) We used to go sailing on the lake in summer.
(3) I didn't use to like him much when we were at school.
(4) You used to see a lot of her.
(5) I used to drink a lot of milk when I was young.

Do, Does
• They are used in the Simple Present Tense or Present Indefinite in affirmative sentences, negative sentences, interrogative sentences and negative-interrogative sentences:-

(1) I do go to school daily.
(2) She does swim in the evening.
(3) They do not obey their elders.
(4) You do not tell a lie.
(5) Do you speak the truth?
(6) Don't you play cricket?

Note that in affirmative sentences 'Do and Does' are used to give emphasis to the main verbs.

I go to school daily.
I do go to school daily.

There is a world of difference between the above two sentences. The first sentence is a general statement where as the second sentence asserts certainty without fail.

Do, Does

Did
• It is used in the Simple Past Tense or Past Indefinite in affirmative sentences, negative sentences, interrogative sentences and negative-interrogative sentences:-

(1) Did the teacher teach English yesterday?
(2) Did I not watch a movie yesterday?
(3) Did you not go to school yesterday?
(4) She did play on the flute yesterday evening.
(5) I did meet her in the street.
(6) He did not make a noise.
(7) They did not play well.

Note that in affirmative sentences 'did' is used to give emphasis to the main verb:

I watched a movie yesterday night.
I did watch a movie yesterday night.

The first sentence is a general statement while the second statement asserts certainty without fail.
'Do, Does and Did' are not modal verbs; they are only auxiliary or helping verbs.

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