a. I didn't realize that you were a cop. or b. I didn't realized that you were a cop.
When two (or more) verbs come one after the other like this in English, the second verb must be an infinitive ("to notice" or "notice") or a participle ("noticing", "noticed"). Only the first verb is conjugated.
- I didn't notice
- I am noticing
- I hadn't noticed
- I don't like | eating vegetables
- I have stopped | trying | to understand