If I . . . . . . . . you, I would not repeat that mistake.

The correct answer is: were

The sentence “If I were you, I would not repeat that mistake” is a conditional sentence. Conditional sentences are used to talk about things that are possible or probable, but not certain. In this case, the speaker is saying that if they were in the listener’s position, they would not repeat the mistake.

The verb “were” is used in the first conditional because it is a past tense verb. The past tense is used in the first conditional to talk about things that are possible or probable, but not certain.

The other options are incorrect because they are not past tense verbs. The verb “is” is a present tense verb, the verb “had been” is a past perfect tense verb, and the verb “are” is a plural form of the verb “to be.”

Here is a table that shows the different types of conditional sentences and the verbs that are used in them:

| Type of conditional sentence | Verb |
| — | — |
| First conditional | Past tense |
| Second conditional | Past perfect tense |