Words "leave" and "arrive" belong to which part of speech (noun or bare infinitive) in the following sentences:
We saw Fred leave.
Did you hear them arrive?
Words "leave" and "arrive" belong to which part of speech (noun or bare infinitive) in the following sentences:
We saw Fred leave.
Did you hear them arrive?
You are asking for the part of speech. They are all verbs functioning as catenative verbs.
Verbs of perception are idiosyncratic in that they can take a bare-infinitival clause as a complement. So both sentences are fine.
Additional information
He made me cry.OK
He made me to cry.not OK
However, make requires a to-infinival complement when it's passive.
He was made to repeat the whole story.
That helps (to) propel me to victory.
We saw Fred leave.
Did you hear them arrive?
These are both catenative constructions.
"See" and "hear" are catenative verbs, and "leave" and "arrive" are bare infinitival non-finite clauses functioning as catenative complements. The intervening objects "Fred" and "them" belong syntactically in the matrix clauses, not the infinitivals, though they are, of course, understood as the subjects of the latter.