Learn French: Il semble fatigué.

Luckily, no! While sembler (to seem) is a verb of perception, it is a completely regular -er verb. It conjugates just like parler or aimer, making it much easier to use than voir and croire.

Since fatigué is an adjective, does it change based on the subject?

Yes! Because sembler is a state-of-being verb, just like être (to be), the adjective following it describes the subject. This means fatigué must agree with the masculine singular subject il. If the subject were feminine, like elle, you would add an e to make it fatiguée.

Could I use a verb like croire to express this same idea?

You cannot swap semble for croit in this exact sentence structure. However, you can express a very similar meaning by using a slightly longer sentence: Je crois qu'il est fatigué (I believe that he is tired).

I've learned the phrase il a l'air before. Does it mean the same thing as il semble?

Yes, they are practically interchangeable! Il a l'air fatigué (He looks tired) and Il semble fatigué (He seems tired) express the exact same idea. Sembler can sound just a tiny bit more formal, but both are extremely common in everyday French.