It's should have + past participle. It's a simple rule...where it becomes more complicated is when you have irregular verbs that have past participles that are different from the regular past tense of the verb (for most verbs they are the same, hence the confusion).

All your examples are of irregular verbs!

1. drunk is the past participle of 'to drink', therefore should have drunk

2. sung is the past participle of 'to sing', therefore should have sung

3. seen is the past participle of 'to see', therefore should have seen

Suggestion: Maybe taking a look at a list of conjugated common irregular verbs. You'll see what I mean about past tense and past participles.

Please let me know if you need more on this.

Thanks Danielle. It is clear now.

-- PL

Cool glad that I could help!