En esa frase falta una comilla al final.

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

So falta una comilla literally means something like one quotation mark is missing.

This is a very common Spanish structure:

• Falta una palabra. = A word is missing.
• Faltan dos páginas. = Two pages are missing.

So in your sentence, falta does not mean he/she lacks in the usual sense. It means that something is absent.

Because the grammatical subject is una comilla, which is singular.

Spanish makes the verb agree with the thing that is missing:

• Falta una comilla. = One quotation mark is missing.
• Faltan dos comillas. = Two quotation marks are missing.

Even though in English we might say there is or there are, in Spanish the verb agrees directly with the missing thing.

Because with verbs like faltar, Spanish very often puts the thing that is missing after the verb.

So:

• Falta una comilla.

is more natural than:

• Una comilla falta.

The second version is possible in special contexts, but it sounds much less normal.

This is similar to other common Spanish patterns:

• Hay un problema.
• Queda un minuto.
• Falta una comilla.

Comilla means quotation mark or quote mark.

Examples:

• comillas = quotation marks
• comilla de apertura = opening quotation mark
• comilla de cierre = closing quotation mark

In your sentence, una comilla is singular because only one mark is missing.

Because the sentence is talking about just one missing mark: the one at the end.

So:

• falta una comilla al final = one quotation mark is missing at the end

If both were missing, you could say:

• Faltan las comillas.
• Faltan dos comillas.

Spanish is being very literal here: only one mark is absent, so singular.

En esa frase means in that sentence/phrase.

The preposition en is used because the missing punctuation is located inside that sentence, or belongs within it.

So the idea is:

• En esa frase = In that sentence
• falta una comilla = a quotation mark is missing

Spanish often uses en to talk about something contained within a text or sentence.

Yes, in some contexts you could also say:

• Falta una comilla al final de esa frase.

That sounds very natural too.

The difference is mostly one of structure and emphasis:

• En esa frase falta una comilla al final.
  = In that sentence, a quotation mark is missing at the end.

• Falta una comilla al final de esa frase.
  = A quotation mark is missing at the end of that sentence.

Both are good Spanish. The original sentence starts by setting the location/topic first: in that sentence.

This is a very common question.

In everyday Spanish, frase is often used loosely for sentence.

But in grammar, there is a distinction:

• frase = phrase / expression / sometimes sentence in everyday use
• oración = sentence/clause with a verb, in more grammatical language

So in strict grammar, oración is often the more precise word for a full sentence. But normal speakers often say frase in casual contexts.

That is why En esa frase falta una comilla al final sounds natural.

Spanish has three common demonstratives:

• esta = this
• esa = that
• aquella = that over there / that more distant one

So esa frase means that sentence.

Very roughly:

• esta frase = this sentence
• esa frase = that sentence
• aquella frase = that sentence over there / that more distant sentence

In real usage, esa is extremely common for that.

Because a + el contracts to al in Spanish.

So:

• a + el final → al final

This contraction is required.

Compare:

• Voy al cine. = I’m going to the cinema.
• Está al final del libro. = It’s at the end of the book.

So al final simply means at the end.

Here al final means at the end.

It refers to the final position in the sentence.

So:

• una comilla al final = a quotation mark at the end

This is a very common expression:

• al principio = at the beginning
• al final = at the end

It is very natural.

Spanish often allows flexible word order, especially when the speaker wants to highlight the setting first.

So:

• En esa frase falta una comilla al final.

sounds natural and clear.

Another natural version is:

• Falta una comilla al final en esa frase.

But that version is less elegant in many contexts.

And this is also very natural:

• Falta una comilla al final de esa frase.

The original sentence puts En esa frase first to establish what sentence you are talking about.

Yes, but that structure is slightly different.

• A esa frase le falta una comilla al final.
• Le falta una comilla al final.

Here le refers to the sentence, as if the sentence is the thing that lacks something.

That is also very common Spanish.

So these are both natural:

• En esa frase falta una comilla al final.
• A esa frase le falta una comilla al final.

The meaning is basically the same, but the structure changes.

Yes, comilla can be a general word for a quotation mark.

If you want to be more specific, Spanish can say:

• comillas dobles = double quotation marks
• comillas simples = single quotation marks
• comillas angulares = angle quotes, like « »

In your sentence, comilla is just the general term.

It works perfectly in Spain Spanish, and it is also understandable in other varieties of Spanish.

A few details:

• comilla is standard
• frase in this kind of everyday use is normal
• the structure with falta is standard across the Spanish-speaking world

So it does not sound strange outside Spain, even if your learning focus is Spanish from Spain.

Because Spanish normally leaves subject pronouns out when they are not needed.

The verb falta already tells you it is third person singular, and the real subject appears later:

• falta una comilla

So Spanish does not need something like ella here.

This is very normal:

• Falta una firma.
• Sobra una palabra.
• Queda un minuto.

The noun after the verb gives the important information.