Nakaupo si Lola sa sala habang nanonood ng telebisyon.

Si is the personal marker used for a singular person when that person is the sentence topic. Since Lola refers to a person and is being treated like a name or title here, si is the natural marker.

Compare:

Si Lola = Grandma / Grandmother as the topic
Ang lola = the grandmother

So si helps show that Lola is the person being talked about.

Here it works very much like a name or title.

Lola literally means grandmother / grandma, but in Filipino family terms are often used as forms of address and can behave almost like proper names. That is why you can say Si Lola.

So in this sentence, it is not just a grandmother in a general sense; it is more like Grandma as a specific person.

Nakaupo describes a state: someone is seated or in a sitting position.

It does not focus on the action of sitting down. Instead, it focuses on the result: the person is already sitting.

So the idea is:

umupo = to sit down
nakaupo = seated / sitting

This is why nakaupo fits well in a scene-setting sentence like this one.

The difference is very important:

Umupo focuses on the action of sitting down.
Nakaupo focuses on the state after that action.

For example:

Umupo si Lola. = Grandma sat down.
Nakaupo si Lola. = Grandma is seated / Grandma is sitting.

So in your sentence, nakaupo tells us her posture, not the moment she moved into that posture.

Sa is the marker commonly used for location.

So:

sa sala = in the living room

The word sala is a very common Filipino word for living room, and it originally came from Spanish. Filipino does not use articles like the or a in the same way English does, so sa sala naturally covers the idea of in the living room.

Habang means while or as. It connects two actions or states that happen at the same time.

In this sentence, it links:

Nakaupo si Lola sa sala
and
nananood ng telebisyon

So habang tells you that the watching is happening at the same time as Grandma being seated in the living room.

Because the subject is already understood to be Lola.

In English, you often repeat the subject:

Grandma was seated in the living room while she was watching television.

In Filipino, once the subject is clear, it is often left out if it would be obvious from context. So after habang, the listener naturally understands that it is still Lola who is watching television.

A fuller version could be:

Nakaupo si Lola sa sala habang siya ay nanonood ng telebisyon.

But that sounds less natural and more explicit than necessary.

Nanonood is the ongoing/imperfective form of the verb based on nood (watch).

A simple set of related forms is:

manood = to watch / will watch
nanood = watched
nanonood = is watching / watching

The repeated syllable helps signal an ongoing action. So nanonood fits the idea that the watching is in progress at that moment.

Because telebisyon is the object of the watching.

In this sentence, Lola is the topic, and the verb is in an actor-focus form: nanonood. In that kind of sentence, the thing being watched is usually marked by ng.

So:

nananood ng telebisyon = watching television

Here, ng does not mean of in the English sense. It is functioning as an object marker.

Not in this sentence as it stands.

With nanonood, the topic is the watcher, which is Lola. Because of that, the thing watched takes ng.

If you wanted television to be the topic instead, the sentence structure and verb form would need to change. So ng telebisyon is the correct choice here.

Because predicate-first word order is very common in Filipino.

So a natural Filipino pattern is:

Nakaupo si Lola...

rather than starting with the subject the way English often does.

You can also say:

Si Lola ay nakaupo sa sala habang nanonood ng telebisyon.

That is grammatical too, but it can sound more formal, more careful, or more written. The original version is very natural and conversational.

It is more about aspect than tense.

Filipino verbs usually emphasize whether an action is:

completed
ongoing
not yet started / contemplated

So here:

nanonood shows an ongoing action
nakaupo shows a state/resulting condition

Rather than marking tense exactly the way English does, the sentence highlights what is currently happening and what state Grandma is in.