Traditionally, a plural noun. Sometimes, "trial" is used colloquially as a verb, meaning "to conduct a trial of", and in that instance, "trials" is the third person singular present indicative of the verb.
"Curfew" is a noun.
Jury is a noun.
Accomplice is a noun.
Custody is a noun.
Yes. There are hundreds of trials in the thousands of courts every day. While more than 90% of cases settle or result in a plea, the sheer number of cases require that they are many trials.
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.
The part of speech for "indefinite" is an adjective.
The part of speech for "buried" is a verb.