Learn Hardcore Portuguese (Portugal): As legendas ajudam quando a série tem falas muito rápidas. - Subtitles help when the series has very fast lines.

Dropping the article (Legendas ajudam) is possible but sounds headline-like or less natural in everyday speech.
Umas legendas would mean “some subtitles,” i.e., a particular, unspecified set.

What exactly does falas mean here? Could I use diálogos or fala instead?

falas = the characters’ spoken lines/utterances (the term used in film/TV scripts).
diálogos = dialogues/conversations (emphasizes interaction; less precise for “lines”).
fala (singular) = one line/speech, or “speech” in a general/linguistic sense.
In this sentence, falas is the most natural choice.

Why is it rápidas? How does the agreement work?

Adjectives agree with the noun’s gender and number:

fala is feminine; falas is feminine plural.
The adjective must match: rápida (sing.), rápidas (pl.).
Hence, falas muito rápidas.

Why is it muito rápidas and not muitas rápidas?

muito here is an adverb modifying the adjective rápidas, so it stays invariable: muito rápidas = “very fast.”
muitas would modify a noun (quantity), e.g., muitas falas rápidas = “many fast lines.” Different meaning.

Why tem and not têm?

tem = 3rd person singular of ter (“has”).
têm (with circumflex) = 3rd person plural (“have”).
Since the subject is singular (a série), you need tem.
Compare: As séries têm falas muito rápidas.

Could I use há instead of tem?

Yes, with a slight nuance:

Quando a série tem falas… highlights that the series “has” those fast lines.
Quando há falas… = “when there are fast lines,” focusing on existence.
Both are fine in Portugal. Note that in Brazil, tem is often used for “there is/are,” but in Portugal há is the standard “there is/are.”

Does quando here mean “when” or “whenever”?

In habitual, general statements, quando can mean “whenever.”

As legendas ajudam quando… ≈ “Subtitles help whenever…”
If you want to be explicit, you can say sempre que: As legendas ajudam sempre que a série tem…

Is rápidas the best way to say “fast” speech? What about depressa or rapidamente?

With the noun falas, you need an adjective: falas rápidas (“fast lines”).
With the verb falar, use an adverb: falar muito depressa (most common in Portugal) or falar rapidamente (more formal).
Natural alternatives:
As legendas ajudam quando falam muito depressa.
As legendas ajudam quando há falas muito rápidas.

Can I change the word order?

Yes. Two common options:

Quando a série tem falas muito rápidas, as legendas ajudam.
As legendas ajudam quando há falas muito rápidas na série.
Both are natural in European Portuguese.

Are there more idiomatic ways to say “help” in this context?

Yes, very natural in Portugal:

As legendas dão jeito quando… (super common, informal–neutral)
Also fine:
As legendas são úteis quando…
As legendas são uma ajuda quando…

Does a série refer to a specific show, or to shows in general?

On its own, a série most naturally refers to a specific show known from context.
To talk about TV shows in general, you’ll more often see the plural: as séries.

General: As legendas ajudam quando as séries têm falas muito rápidas.
Context can still make the singular generic, but the plural is clearer for general statements.

What’s the term for “subtitles” in Portugal? Is there a difference from “closed captions”?

legendas = subtitles (translation or same-language subtitles, depending on context).
“Closed captions” (with sound/speaker info) are usually called legendas para surdos (often abbreviated LSE) or legendagem oculta. The plain legendas typically don’t include sound descriptions.

Any quick pronunciation tips (Portugal)?

j in ajudam/legendas = “zh” (as in “measure”).
Final s after a vowel (e.g., legendas, rápidas, falas) sounds like “sh.”
muito ≈ “MOOY-to.”
quando ≈ “KWAN-doo.”
série has stress on the first syllable: “SEH-ree-eh” (the final vowel is very short).
These are approximations, but they’ll get you close to European Portuguese pronunciation.