Sa pagdating ng mga bisita, binuksan ko ang pinto.

Literally, it means At the arrival of the guests or Upon the guests’ arrival.

• sa = at, in, on, upon
• pagdating = arrival, coming
• ng mga bisita = of the guests / the guests’

So this whole first part is a time expression: When the guests arrived or Upon the guests’ arrival.

Because pagdating is being used as a noun-like form, not as a regular finite verb.

• dumating = arrived
• pagdating = arrival / arriving

Filipino often uses pag- forms to turn actions into event nouns or time expressions. So:

• dumating ang mga bisita = the guests arrived
• sa pagdating ng mga bisita = at the arrival of the guests / when the guests arrived

This structure is very common in Filipino.

Here, ng mga bisita tells us whose arrival it is.

So:

• pagdating = arrival
• ng mga bisita = of the guests

Together: the arrival of the guests

This ng is not marking a direct object here. It is linking mga bisita to pagdating, almost like possession or association.

Sa is marking the whole phrase as a time setting.

In this sentence, sa pagdating ng mga bisita means something like:

• when the guests arrived
• upon the arrival of the guests
• at the guests’ arrival

So sa helps turn the phrase into a time expression that sets the scene for the main action.

Mga is the plural marker.

• bisita = guest / visitor
• mga bisita = guests / visitors

Filipino nouns usually do not change form for plural, so mga is what shows that the noun is plural.

This is about focus/voice, which is a central part of Filipino grammar.

• Binuksan ko ang pinto puts focus on ang pinto.
• Nagbukas ako ng pinto puts focus on ako as the doer.

In your sentence:

• binuksan = opened, in object-focus form
• ko = by me / I
• ang pinto = the door

So Binuksan ko ang pinto is literally structured more like The door was opened by me, but in natural English it is still just I opened the door.

It comes from the root bukas, meaning open.

A simplified breakdown is:

• bukas = open
• buksan = open something
• binuksan = opened something

The form binuksan uses the infix -in-, which commonly marks completed aspect in this verb pattern.

So binuksan means the opening action has been completed.

Because ko is a short pronoun, and short pronouns in Filipino often come very early in the sentence, usually after the first word or predicate.

So:

• binuksan ko ang pinto

not usually

• ko binuksan ang pinto in a neutral statement

Here, ko means I or by me, depending on how you think about the structure. In this object-focus sentence, ko marks the actor.

Ang pinto is the focused or topic noun in the sentence.

• ang marks the noun phrase that is in focus/topic position
• pinto = door

Because the verb is binuksan, the thing being opened is marked with ang:

• Binuksan ko ang pinto = I opened the door

So even though door is the object in English meaning, it is the ang-marked noun in Filipino because of the verb form used.

Yes. Filipino word order is fairly flexible.

For example, you could also say:

• Binuksan ko ang pinto sa pagdating ng mga bisita.

This still means basically the same thing. The difference is mostly what you want to emphasize.

• Sa pagdating ng mga bisita, binuksan ko ang pinto.
  This foregrounds the time setting.
• Binuksan ko ang pinto sa pagdating ng mga bisita.
  This foregrounds the action first.

Yes, very often it can.

• Pagdating ng mga bisita, binuksan ko ang pinto.
• Sa pagdating ng mga bisita, binuksan ko ang pinto.

Both are understandable. The version without sa is very common and natural in everyday Filipino. The version with sa can sound a bit more formal or explicit.

Not really. Pagdating is not a regular tense-marked verb here. It is more like arrival or arriving.

The past meaning of the whole sentence mainly comes from:

• the context
• the completed verb binuksan

So in this sentence, we naturally understand that the guests arrived and then the speaker opened the door.

Yes, it is natural and grammatical.

A very common alternative would be:

• Pagdating ng mga bisita, binuksan ko ang pinto.

That version may sound a little more conversational, but your original sentence is perfectly fine.