A main clause contains at least a subject and a conjugated verb. "und" and "oder" are conjunctions that can connect two independent clauses.
| No conjunction: | Ich bin in Deutschland. | Ich mache Urlaub. | |||
| Bist du dumm? | Bist du klug? | ||||
| With a conjunction: | Ich bin in Deutschland | und | ich mache Urlaub. | ||
| Bist du dumm | oder | bist du klug? |
If the two clauses share the same subject, then the subject can be omitted from the second main clause. That's shorter:
| Shortened form: | Ich bin in Deutschland | und | mache Urlaub. |
If the subject and conjugated verb are identical in each of the main clauses, then both can be dropped from the second clause.
| Shortened form: | Bist du dumm | oder | klug? |