Kapag nakaupo na ang lahat sa upuan, tatawid na ang bus sa tulay malapit sa ilog.

Kapag introduces a time/condition clause, usually when, once, or whenever.

In this sentence, it means something like once / when everyone is seated.

Common comparison:

• kapag = when / once / whenever
• pag = a shorter, more conversational form of kapag
• kung = if, a more general condition

So kapag is a good choice here because the event is expected to happen after that condition is met.

Because the sentence focuses on the state of being seated, not the action of sitting down.

• nakaupo = seated / sitting
• umupo = to sit down / sat down
• naupo = got seated / ended up sitting

So nakaupo na ang lahat means everyone is already seated, not everyone sat down.

Na is a very common particle with meanings like already, now, by then, or at that point. Its exact feel depends on context.

Here:

• nakaupo na ang lahat = everyone is already seated
• tatawid na ang bus = the bus will then / now cross

So the first na marks that the seated state has been reached, and the second na marks the next step that follows.

Not exactly. Ang is not just the English article the. It marks the main noun phrase of the clause, often called the topic or subject-like phrase.

Here:

• nakaupo na is the predicate
• ang lahat is the topic: everyone / all of them

So ang lahat means everyone in this context.

Upuan means seat or chair. It comes from the root upo (sit) plus -an, which often makes a place for something.

So:

• upo = sit
• upuan = seat / place to sit

As for the singular form, Filipino often uses a singular noun in a generic or collective way. So sa upuan can still mean people are seated in their seats. If you want to make the plural extra explicit, you could say sa mga upuan.

Tatawid is the contemplated/future aspect form of the verb from the root tawid (cross).

Related forms:

• tumawid = crossed
• tumatawid = is crossing / crosses
• tatawid = will cross

Filipino is often described as marking aspect more than tense, but for learners, tatawid is fine to understand as will cross.

Yes, you can say Ang bus ay tatawid..., but the original order is very normal in Filipino.

A common Filipino pattern is:

• predicate first
• then the ang-marked phrase

So:

• tatawid na ang bus = natural, common order
• ang bus ay tatawid = also correct, often a bit more formal or more explicitly topic-first

English usually puts the subject first, but Filipino often does not.

Because sa tulay presents the bridge as a location or path: the bus crosses on / across / by way of the bridge.

With a verb like tumawid / tatawid, you may also hear ng when the thing crossed is treated more directly as an object:

• tumawid ng ilog = crossed a river
• tumawid ng tulay = crossed a bridge

But sa tulay is very natural when the idea is movement along that route or at that location.

Most naturally, it modifies tulay.

So the most likely reading is:

• the bridge near the river

That is because in Filipino, a descriptive phrase often comes right after the noun it modifies:

• sa tulay malapit sa ilog = at/on/across the bridge near the river

Malapit means near or close.

With sa, it forms a phrase:

• malapit sa ilog = near the river

Here it acts like a modifier describing tulay. Filipino often uses adjective-like words this way after the noun.

In this sentence, it sounds like one future event, mainly because of tatawid.

So the idea is:

• once everyone is seated, the bus will cross

But kapag can also be used for repeated or habitual situations. If the main verb were different, the whole sentence could mean whenever everyone is seated, the bus crosses.

So kapag itself does not force only one meaning; the verb forms help show whether the sentence is future or habitual.