Didn't? Confused... Are singular verbs take singular subject right?

Honestly, I was terribly confused using Didn’t or did not, but normally, I believed that singular verbs take singular subject right?

like
1.He doesn’t like it?
2, He believes in reality.

what if : He didn’t believe in reality.

why is it that some American uses like…

  1. Hey she[color=blue] didn’t say that one.

I’m confused…help please…

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‘Did’ has no ‘-s’ form. ‘Didn’t’ and ‘did not’ are the same.
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Hi Sanunai

To form a negative sentence in the simple present tense, you use either do or does, the word not, and the base form of the verb. The word does is used for the third person singular (he/she/it) in the simple present tense.

All of these sentences are simple present tense and third person singular:

  1. He doesn’t like it.
  2. He believes in reality.
  3. It doesn’t make sense.
  4. Sally lives in the USA.
  5. The doorbell doesn’t work.

[color=white].
To form a negative sentence in the simple past tense, you use did, the word not, and the base form of the verb. The word did is used for first, second and third person singular and plural.

All of these sentences are simple past tense and third person singular:

a. She didn’t say that.
b. It didn’t rain yesterday.
c. It rained last Thursday.
d. Susan didn’t make any mistakes on the test last week.
e. Bill made a lot of mistakes on the test last week.
f. Sally visited London two years ago.
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thank you so much Amy, big help for me anyway I will ask some or maybe next day, MABUHAY all the way from Philippines, A million Thanks and other facilitators