Learn Russian: Роман написан известным критиком.

In Russian passive sentences, the agent (the person doing the action) is shown just by putting the noun into the Instrumental case, with no preposition. That is why известным критиком is in the Instrumental case, which naturally translates to "by a famous critic" in English.

Why do we use написан instead of the regular past tense написал?

Because this is a passive sentence. If you used написал, the sentence would literally mean that the novel itself wrote something! написан is a short-form passive participle. It describes the subject (роман) as being in a "written" state.

Why doesn't известным have the same -ом ending as критиком since they are both Instrumental masculine?

Adjectives and nouns follow different declension rules. критик is a masculine noun ending in a consonant, which takes -ом in the Instrumental case. известный is an adjective, and masculine adjectives take -ым (or -им) in the Instrumental.

The translation says "is written". Could this also mean "was written"?

Yes! Short-form perfective participles like написан describe a result that exists now, which often translates to "is written", but it can also mean "was written". If you want to strictly emphasize that the writing happened in the past, you would add the past tense of "to be" and say Роман был написан.