Learn French: Je sais que cette œuvre est appréciée par les critiques. - I know that this work is appreciated by the critics.

In the passive voice, the past participle acts like an adjective and must agree in gender and number with the subject. Since cette œuvre (this work) is feminine singular, we add an e to make it appréciée.

Why do we use par here instead of de to mean 'by'?

In the passive voice, the agent (the one doing the action) is typically introduced by par. While some verbs of state or emotion can use de, apprécié uses par when referring to specific people taking an active role, like les critiques here.

How do I know if œuvre is masculine or feminine?

The word œuvre is feminine. This sentence gives you two big clues: it uses the feminine demonstrative adjective cette (this), and it forces the feminine spelling on the past participle appréciée.

Could I use Je connais instead of Je sais here?

No, you need savoir (conjugated here as Je sais) because it is followed by que (that) to state a fact. You use connaître when you are saying you are familiar with a noun, like a specific person, place, or book.

Does les critiques mean 'the reviews' or 'the critics'?