Learn Portuguese (Portugal): O ladrão está preso.

In Portuguese, being under arrest or in prison is viewed as a temporary state or condition, not an inherent part of someone's identity. Because of this, we use estar (the verb for temporary states) rather than ser.

I know the verb for "to arrest" is prender. Why is the word here preso and not prendido?

The verb prender has two past participles (the form of a verb used to mean "arrested"). The regular form prendido is used with the verb ter for compound tenses. However, when used as an adjective to describe a state, which requires the verbs ser or estar, you must use the irregular, short form preso.

Could está preso mean anything else besides being under arrest?

Yes! Because the verb prender broadly means "to attach", "to tie", or "to trap", the phrase está preso is very commonly used in everyday life to mean "is stuck" (like a jammed zipper or door) or "is attached" (like a dog on a leash).

If there were multiple thieves, how would this sentence change?