Learn Italian: Il sospettato è colpevole di questo crimine.

Sospettato is the past participle of the verb sospettare (to suspect). Just like in English, Italian often uses a past participle as a noun to describe a person. So, il sospettato literally means "the suspected person" or "the suspect".

Does colpevole always use the preposition di?

Yes! When you want to say someone is "guilty of" something, Italian uses colpevole di. It matches the English structure perfectly.

Crimine ends in "-e". How can I tell its gender from this sentence?

Nouns ending in -e can be either masculine or feminine, so their gender usually has to be memorized. However, you can tell crimine is masculine here because it is paired with questo (this). If it were feminine, it would be paired with questa.

How would I say "The suspects are guilty"?

You would say I sospettati sono colpevoli. Remember that both the noun and the adjective must agree in number. Since colpevole is an adjective ending in -e, its plural form ends in -i.