Mataas ang buwanang bayad.

Filipino often puts the predicate first. In this sentence, mataas is the predicate, meaning something like high.

So the structure is:

• Mataas = the description/predicate
• ang buwanang bayad = the thing being talked about

A very literal structure is:

• High is the monthly fee

But in natural English, that becomes The monthly fee is high.

There is no separate word for is here. In simple present-tense Filipino sentences like this, the linking verb is/are is often not expressed.

So:

• Mataas ang buwanang bayad.
• literally: High the monthly fee
• naturally: The monthly fee is high

This is completely normal in Filipino.

Ang marks the topic or the noun phrase being identified or described. Here, it marks buwanang bayad.

So:

• ang buwanang bayad = the monthly fee/payment as the topic of the sentence

It is sometimes tempting to translate ang directly as the, but that is not always exact. Its grammatical job is broader than just being an article.

Buwanang means monthly.

It comes from buwan, which means month, and it functions here as a modifier for bayad.

So:

• buwan = month
• buwanang bayad = monthly payment / monthly fee

In this sentence, buwanang tells you that the bayad happens every month.

Filipino usually needs a link between a modifier and the word it modifies. In buwanang bayad, buwanang is the proper linked form used before the noun bayad.

So the phrase works as a unit:

• buwanang bayad = monthly fee/payment

You should learn buwanang here as the normal form meaning monthly before a noun.

In this sentence, bayad means payment, fee, or charge, depending on context.

So buwanang bayad could be translated as:

• monthly payment
• monthly fee
• monthly charge

The best English word depends on the situation. For example:

• for tuition or service plans, monthly fee/charge
• for installments, monthly payment

Because mataas is not directly modifying bayad inside one noun phrase. It is the predicate of the whole sentence.

Compare these:

• Mataas ang buwanang bayad. = The monthly fee is high.
• mataas na buwanang bayad = a high monthly fee

In the second example, mataas directly modifies bayad, so you need the linker na.

In your sentence, mataas stands separately as the predicate, so no na is used.

Yes. That is also correct.

Compare:

• Mataas ang buwanang bayad.
• Ang buwanang bayad ay mataas.

Both mean the same thing: The monthly fee is high.

The first version is very natural and common because Filipino often uses predicate-first order.
The second version can sound a bit more formal, deliberate, or contrastive.

Yes. Mataas literally means high, and Filipino commonly uses it for amounts such as:

• prices
• fees
• salaries
• rates
• bills

So mataas ang bayad naturally means:

• The fee is high
• The payment is expensive/high

This is similar to English expressions like high price or high cost.

Yes. It is a complete sentence.

This is an adjectival or descriptive sentence, not an action sentence. Filipino does not need an action verb here, because the sentence is simply describing something:

• Mataas = the description
• ang buwanang bayad = what is being described

So it is fully grammatical and complete on its own.