Libre ang isang tasa ng tsaa kapag bumili ka ng tinapay.

Filipino often leaves out the present-tense to be in sentences like this.

So Libre ang isang tasa ng tsaa literally looks like Free the/a cup of tea, but it naturally means A cup of tea is free.

This is very common with adjectives and noun-based predicates in Filipino.

Because Filipino very often puts the predicate first.

In Libre ang isang tasa ng tsaa, the thing being said about the topic is libre. The topic is ang isang tasa ng tsaa.

So the structure is basically:

• Libre = free
• ang isang tasa ng tsaa = the/a cup of tea

A more topic-first version is also possible:

Ang isang tasa ng tsaa ay libre kapag bumili ka ng tinapay.

That sounds a bit more formal or deliberate. The original sentence sounds natural and ad-like.

Ang marks the topic of the clause.

In this sentence, ang isang tasa ng tsaa is the thing being talked about: it is the thing that is free.

Important: ang is not exactly the same as English the. Sometimes it can line up with the, but its main job is grammatical: it marks the topic or focused noun phrase.

It can work as either, depending on context.

Isang is formed from isa + a linker form -ng before the next word.
So:

• isa = one
• isang tasa = one cup / a cup

Since Filipino does not have articles that work exactly like a/an, isang is often used where English would say a.

So isang tasa ng tsaa can mean:

• one cup of tea
• a cup of tea

Here, English would usually say a cup of tea.

Here, ng links tasa and tsaa to make cup of tea.

So:

• tasa = cup
• tsaa = tea
• tasa ng tsaa = cup of tea

This ng is very common in expressions like:

• baso ng tubig = glass of water
• hiwa ng tinapay = slice of bread
• mangkok ng kanin = bowl of rice

So in this part of the sentence, ng is functioning like of.

It can feel like both.

Kapag is commonly used for a condition or situation that is expected, repeated, or tied to a future event. In a sentence like this, it often means when or if in the sense of a store offer or general rule:

Kapag bumili ka ng tinapay...
= When/If you buy bread...

In promotional language, English might translate it more naturally as:

• when you buy bread
• if you buy bread
• with the purchase of bread

All are close in meaning here.

This is a very common learner question.

Bumili comes from the root bili (buy) with -um-, an actor-focus verb marker.

The tricky part is that for many -um- verbs, the form bumili can look the same as the basic verb form and the completed form. In a kapag clause, Filipino commonly uses this form to mean a future or conditional event:

kapag bumili ka ng tinapay
= when/if you buy bread

So even though bumili may remind you of a past action in some contexts, here it does not mean when you bought. The word kapag makes the meaning conditional/future-like.

Ka is the short form of you, and it is normally used after verbs or particles.

So:

• bumili ka = you buy / you bought / buy
• ikaw = the full form of you

In this sentence, kapag bumili ka is the natural everyday order.

If you used ikaw, you would normally need a different structure, such as:

kapag ikaw ay bumili ng tinapay

That is grammatical, but more emphatic or formal.

Because tinapay is the object of bumili in this structure.

With an actor-focus verb like bumili, the thing being bought is usually marked by ng:

• bumili ka ng tinapay = you bought / buy bread

If tinapay were marked with ang, the verb structure would normally change too. For example:

• Binili mo ang tinapay = You bought the bread

So in the original sentence, ng tinapay is the expected pattern.

Yes. That is completely natural.

Both of these work:

• Libre ang isang tasa ng tsaa kapag bumili ka ng tinapay.
• Kapag bumili ka ng tinapay, libre ang isang tasa ng tsaa.

The difference is mostly about emphasis.

• Starting with Libre highlights the offer right away, which sounds very natural in ads or signs.
• Starting with Kapag bumili ka... highlights the condition first.

Here, libre functions like an adjective meaning free.

It does not change form for number or gender, so the same word is used regardless of what noun it describes.

Examples:

• Libre ang kape. = The coffee is free.
• Libre ang mga inumin. = The drinks are free.

So libre stays libre.