Learn Swedish: Han hittar ett krossat fönster i huset. - He finds a smashed window in the house.

Because it is describing fönster, which is an ett word (ett fönster). When we use a past participle like an adjective, it has to match the noun's gender. Since it's an ett word, the base form krossad changes to krossat.

Is krossat acting as a verb in this sentence?

No, even though it comes from the verb krossa (to smash), here it acts as an adjective describing the window. When a verb is used this way, it is called a past participle. The actual working verb in this sentence is hittar (finds).

How would the sentence change if he found the smashed window?

You would say Han hittar det krossade fönstret i huset. Just like regular adjectives, past participles take an extra ending when used with definite nouns. For participles that end in -ad, the definite ending is -ade.

Could I use trasigt instead of krossat?

Yes, but it changes the meaning slightly. Ett trasigt fönster just means a broken window (it might just be cracked or have a broken frame). Ett krossat fönster specifically means the glass is smashed or shattered.