Gamitin mo ang susi ko para buksan ang pinto.

A very rough breakdown is:

• Gamitin mo = Use
• ang susi ko = my key
• para = in order to / to
• buksan = open
• ang pinto = the door

So the sentence is literally something like Use my key in order to open the door.

Gamitin is a verb form from gamit (use, thing used, tool). In this sentence, gamitin means use it / use something.

It is the form commonly used when the thing being used is explicitly mentioned:

• Gamitin mo ang susi ko. = Use my key.

So gamitin is not just the dictionary idea to use; here it is specifically the command form use.

Because mo is a short pronoun, and these short pronouns often come right after the first word or first element of the clause.

So:

• Gamitin mo = Use
• not usually Mo gamitin

Here, mo means you. In this kind of sentence, it marks the person being told to do the action.

Not exactly. Ang is a marker, and although it is often translated as the, it does not work exactly like the English article the.

In this sentence:

• ang susi ko = the marked noun phrase my key
• ang pinto = the marked noun phrase the door

A useful beginner idea is:

• sometimes ang looks like the
• but grammatically it is a Filipino marker, not a direct one-to-one equivalent of English the

That is why ang susi ko does not mean a clumsy literal the my key. It just means my key in the role required by the sentence.

Because in Filipino, possessive pronouns usually come after the noun.

So:

• susi ko = my key
• bahay mo = your house
• kotse niya = his/her car

This is the opposite of English word order.

Not always. Ko is a short pronoun that can have different roles depending on where it appears.

After a noun, it is usually possessive:

• susi ko = my key

In other contexts, it can correspond to I, me, or my, depending on the sentence structure.

In this sentence, because it follows susi, it clearly means my.

Para means for, so that, or in order to.

Buksan means to open something.

So:

• para buksan ang pinto = to open the door / in order to open the door

This part explains the purpose of using the key.

Because buksan is the form commonly used when a specific thing is being opened is stated right after it.

Here, the thing being opened is:

• ang pinto = the door

So:

• buksan ang pinto = open the door

By contrast, magbukas is a different verb form and does not fit this exact structure as naturally when you are directly highlighting the thing being opened.

Because ang pinto is the noun phrase connected with buksan.

In other words:

• first part: Gamitin mo ang susi ko = Use my key
• second part: para buksan ang pinto = to open the door

Each verb phrase has its own noun phrase, so seeing ang again is normal.

Yes, it is natural.

A very common alternative is:

• Buksan mo ang pinto gamit ang susi ko.

That also means Use my key to open the door, but it starts with open the door, so the focus feels slightly different.

Compare:

• Gamitin mo ang susi ko para buksan ang pinto.
  Emphasis starts with use my key
• Buksan mo ang pinto gamit ang susi ko.
  Emphasis starts with open the door

Both are good.

Mo is for you singular in ordinary speech.

If you want to be polite, or if you are speaking to more than one person, use ninyo. With extra politeness, add po.

Examples:

• Gamitin ninyo ang susi ko para buksan ang pinto.
• Gamitin po ninyo ang susi ko para buksan ang pinto.

That makes the sentence more polite and/or suitable for plural you.

If you leave out para, the purpose relationship becomes less clear.

• Gamitin mo ang susi ko para buksan ang pinto. = Use my key to open the door.

Without para, it may sound more like separate instructions rather than one action done for a purpose.

So in this sentence, para is helpful and natural because it clearly means to / in order to.