Falta uma caneta na mochila.

Here, falta comes from faltar, which often means to be missing, to be lacking, or to be absent.

In this sentence, it does not mean to miss in the emotional sense, like I miss you. It means that something that should be there is not there.

So falta uma caneta means that a pen is missing.

Because the verb agrees with uma caneta, which is singular.

Even though uma caneta comes after the verb, it is still the grammatical subject here. Since it is singular, the verb is singular too:

• Falta uma caneta. = One pen is missing.
• Faltam duas canetas. = Two pens are missing.

This is a very common pattern with faltar.

The subject is uma caneta.

That can feel strange to English speakers because the subject comes after the verb here. But in Portuguese, especially with verbs like faltar, that is very normal.

So the structure is basically:

• Falta = is missing
• uma caneta = a pen
• na mochila = in the backpack

With faltar, Portuguese very often places the thing that is missing after the verb.

So Falta uma caneta is more natural than trying to force a more English-like order.

This verb often works in a pattern like:

• Falta dinheiro.
• Falta tempo.
• Falta uma página.

Portuguese speakers are very used to hearing the missing thing after falta or faltam.

Na is a contraction of em + a:

• em = in, on, at
• a mochila = the backpack
• em a mochila becomes na mochila

So na mochila means in the backpack.

This contraction is mandatory in normal Portuguese:

• no carro = in the car
• na bolsa = in the bag
• nos livros = in the books
• nas caixas = in the boxes

Because that means something different.

• Tem uma caneta na mochila. = There is a pen in the backpack.
• Falta uma caneta na mochila. = A pen is missing in/from the backpack.

So tem tells you that the pen is there, while falta tells you that the pen is not there even though it should be.

That difference is very important.

Yes. That is very common in Brazilian Portuguese.

• Falta uma caneta na mochila.
• Está faltando uma caneta na mochila.

Both are natural. The version with está faltando can sound a little more conversational or a little more focused on the current situation.

In everyday speech, Brazilians often use both patterns.

Uma caneta means a pen or one pen: an unspecified pen.

If you say a caneta, you mean the pen, a specific pen that both speaker and listener can identify.

Compare:

• Falta uma caneta na mochila. = A pen is missing from the backpack.
• Falta a caneta na mochila. = The pen is missing from the backpack.

So the choice between uma and a depends on whether the pen is specific or not.

You change the noun phrase and make the verb agree with it.

Examples:

• Faltam duas canetas na mochila. = Two pens are missing in/from the backpack.
• Faltam canetas na mochila. = Pens are missing in/from the backpack.

The key point is:

• singular subject → falta
• plural subject → faltam

Yes, it is natural.

It sounds like something you might say when checking school supplies, inventory, or the contents of a bag. In casual spoken Brazilian Portuguese, many speakers might also say:

• Está faltando uma caneta na mochila.

But Falta uma caneta na mochila is perfectly good Portuguese.

In practice, it points to the backpack as the place/context where the absence is noticed.

So the idea is that, when looking at the backpack and what it should contain, one pen is missing.

In English, depending on context, you might express that as either:

• in the backpack
• from the backpack

Portuguese often just uses na mochila naturally in this kind of sentence without needing to spell out that distinction as strongly as English sometimes does.