Alter, Alters, Altering are all different forms of "Alter" in the present tense.
"They" is a pronoun. It cannot have a past tense, as it is a word for a group of people or objects, and not an action / verb that can alter from the present tense to the past.
Altered is the past tense of alter.
The verb is is the present tense.
The past tense of did is did. The present tense of did is do. The future tense of did is will do.
It was, (past tense) it is, (present tense) it will be( future tense)
Past tense I had Present tense I have Future Tense I will have
"Has" is in present tense. The past tense is "had".
No 'is' is present tense. am/is/are = present tense was/were = past tense
Present tense
the present of did is didnt Do is the present tense of did. Didn't is the past negative form
Creep is present tense. The past tense is crept.
Should is present tense.