Umarte siya.

It is built from the root arte plus the infix -um-:

arte → umarte

The -um- is inserted after the first consonant of the root. This is a very common Filipino verb pattern.

In many Filipino verbs, -um- marks an actor-focus verb. That means the sentence is grammatically centered on the doer of the action.

In Umarte siya, the doer is siya. So the structure is focused on he/she as the one doing the acting.

Because predicate-first word order is very common in Filipino. A simple sentence often starts with the verb or predicate, then the subject-like element follows.

So:

Umarte siya. = normal, natural order

English usually prefers He/She acted, but Filipino commonly puts the verb first.

Siya is a third-person singular pronoun. It can mean he, she, or sometimes it, depending on context.

Filipino pronouns usually do not mark gender the way English does, so you need context to know whether the person is male or female.

Because pronouns already have their own special forms. With common nouns, you might see ang or si, but personal pronouns do not need those markers in this position.

For example:

Umarte ang bata. = The child acted.
Umarte si Maria. = Maria acted.
Umarte siya. = He/She acted.

So siya already functions like the ang/si form of the pronoun.

For -um- verbs, umarte can be the basic form and also the completed aspect, depending on context. In a full sentence like Umarte siya, the most natural reading is usually completed: the action happened.

So it is commonly understood as He/She acted or He/She put on an act.

This is one of those cases where context helps decide the exact interpretation.

You change the verb form:

Umaarte siya. = He/She is acting.
Aarte siya. = He/She will act.

So the forms are:

umarte = basic/completed
umaarte = ongoing/incomplete
aarte = contemplated/future

Yes. Umarte can stand on its own, so Umarte siya. is a complete sentence.

If needed, you can add more information, such as:

Umarte siya sa entablado. = He/She acted on stage.
Umarte siya bilang doktor. = He/She acted as a doctor.
Umarte siya nang parang bata. = He/She acted like a child.

But the short sentence by itself is perfectly normal.

Not as the normal neutral sentence. The usual order is:

Umarte siya.

If you want to emphasize siya, Filipino normally uses a different structure, such as:

Siya ang umarte. = He/She was the one who acted.

So Siya umarte by itself does not sound like standard neutral Filipino.

A simple pronunciation guide is:

u-MAR-te si-YA

The stress is on mar in umarte and on ya in siya. The r is usually a light tap, not a strong English r.

So it sounds roughly like:

u-MAR-te si-YA