En la última frase faltaba otra coma, y por eso la idea sonaba menos clara.

Here, faltaba comes from faltar, which often means to be missing.

So faltaba otra coma literally means another comma was missing.
In natural English, you might also understand it as:

• the sentence needed another comma
• there should have been another comma

A very important point is that the grammatical subject is otra coma. Spanish often expresses this idea differently from English: instead of saying someone forgot a comma, it says a comma was missing.

Faltaba is the imperfect tense, while faltó is the preterite.

In this sentence, the speaker is describing the state of the text: at that moment, another comma was missing. The imperfect is natural for describing background, situations, or how something was.

So:

• faltaba = it was missing / there was one missing
• faltó = it was missing at one specific completed moment, or it got omitted

Here, faltaba sounds more natural because the sentence is about how the phrase was written and how it sounded, not about one completed event.

In Spanish, otro/otra normally already includes the idea of another, so you usually do not add un/una before it.

So:

• otra coma = another comma
• una otra coma = not standard in normal Spanish

This is different from English, where another historically contains an + other, but speakers do not think about it that way anymore. In Spanish, otra works by itself.

En la última frase means in the last sentence/phrase.

Spanish uses en here to locate something within a text or inside a sentence. So it is like saying:

• in the last sentence
• in the final phrase

This is very natural in Spanish. The adjective última agrees with frase, which is feminine:

• la frase
• la última frase

It can mean either, depending on context.

For learners, this can be confusing because English separates sentence and phrase more clearly. In Spanish:

• frase can mean a sentence, an expression, or a piece of wording
• oración is the more technical grammatical word for sentence
• frase is very common in everyday language

So in this sentence, frase most likely means sentence, but the exact English translation depends on the context.

Normally, Spanish does not use a comma before y when simply joining two elements.

However, a comma before y can appear when:

• the second part is a separate clause
• there is a pause for clarity
• the writer wants to mark an explanatory or consequential idea

Here, y por eso introduces a result: and because of that / and so. The comma helps separate the two ideas:

• another comma was missing
• because of that, the idea sounded less clear

So while Spanish usually avoids a comma before y, in sentences like this it can be perfectly natural.

Por eso means because of that, for that reason, or that’s why.

It connects the second clause to the first one by showing a result or consequence.

So:

• faltaba otra coma = another comma was missing
• y por eso = and because of that / and that’s why
• la idea sonaba menos clara = the idea sounded less clear

Very common equivalents are:

• por eso = because of that / that’s why
• por lo tanto = therefore
• así que = so

Among these, por eso sounds very natural and conversational.

Again, this is the imperfect.

Sonaba describes how the idea sounded or came across at that time. It gives a descriptive, ongoing sense rather than presenting one completed event.

Compare:

• sonaba menos clara = it sounded less clear
• sonó menos clara = it sounded less clear on a specific occasion or at a specific moment

Because the speaker is describing the quality of the sentence as written, sonaba fits better.

This is a nuance question.

• era menos clara = it was less clear
• sonaba menos clara = it sounded less clear

With sonaba, the speaker focuses on the impression produced by the wording. It is about how the idea came across to the reader or listener.

With era, the speaker would be stating more directly that the idea itself was less clear.

So sonaba menos clara suggests:

• the wording made it seem less clear
• the sentence gave a less clear impression

That is a very natural choice when talking about style, punctuation, or phrasing.

Because clara agrees with la idea, which is feminine singular.

Agreement in Spanish is very important:

• la idea = feminine singular
• clara = feminine singular adjective

So:

• la idea sonaba menos clara
• not la idea sonaba menos claro

If the noun were masculine, you would use claro instead:

• el mensaje sonaba menos claro

Yes, you could, but faltaba otra coma is more natural here.

There is a slight nuance:

• otra coma = another comma
• una coma más = one more comma

Both are understandable, but otra coma sounds smoother and more idiomatic in this sentence. It suggests an additional comma was needed beyond the ones already present.

So:

• faltaba otra coma = very natural
• faltaba una coma más = possible, but slightly more focused on quantity than on the idea of an additional missing comma