In comparative structures with as and than

Words are often left out after as and than, if the meaning is clear.

We scored more runs than them. (= We scored more runs than they scored.)
The weather isn’t as good as last year. (= The weather isn’t as good as it was last year.)

After question words

Clauses can be left out after question words.

Somebody has stolen my watch, but I don’t know who. (= …I don’t know who has stolen my watch.)

Relative pronouns
The relative pronoun that is often dropped in an informal style.

I thought you would come. (= I thought that you would come.)

Be after conjunctions
Subject pronouns (he, she, they, you, it, we) and be (is, am, are, was, were) are sometimes left after certain conjunctions.

Though intelligent, he didn’t receive formal education. (= Though he was intelligent, he didn’t receive formal education.)
While in Rome, do as Romans do. (= While you are in Rome, do as Romans do.)
She had a massive heart attack while asleep. (=…while she was asleep.)

Prepositions
Prepositions are often dropped in a few time expressions. This is common in an informal style.

What time does the train leave? (More natural than ‘At what time does the train leave?’)
See you next Monday. (More natural than ‘See you on next Monday.’)