Learn Hardcore German: Ich frage meine Freundin, welche Serie heute läuft. - I ask my friend which series is on today.

The verb fragen (to ask) takes a direct object in German:
- jemanden fragen = to ask someone
The person you ask is therefore in the accusative.

You can see the change with masculine nouns more clearly:

Ich frage meinen Freund. (accusative: meinen Freund)
Ich frage meine Freundin. (accusative: meine Freundin – same form as nominative for feminine)

So: meine Freundin is the direct object of frage, hence accusative.

Why does the word order change to welche Serie heute läuft and not welche Serie läuft heute?

Because the second part of the sentence is an indirect question, which is a subordinate clause in German.

In a normal main clause (direct question):
- Welche Serie läuft heute? (verb in 2nd position)
In an indirect question (subordinate clause after a comma):
- …, welche Serie heute läuft. (conjugated verb goes to the end)

General rule:
In German subordinate clauses, the conjugated verb goes to the final position in the clause.

What is the comma doing here? Could I leave it out?

The comma is obligatory in German.

Ich frage meine Freundin, welche Serie heute läuft.

It separates the main clause (Ich frage meine Freundin) from the subordinate clause (welche Serie heute läuft).

In contrast to English, German requires a comma before most subordinate clauses introduced by words like dass, weil, ob, welche, wann, wie, warum, etc.
So you cannot leave it out in correct written German.

Why is it welche Serie and not welcher Serie or welches Serie?

Because Serie is:

feminine
singular
in the nominative case in that clause

The forms of welch- in the nominative singular are:

welcher – masculine
welche – feminine
welches – neuter

Since Serie is feminine and functions as the subject of läuft, you need welche Serie.

Why is welche Serie in the nominative case and not accusative?

Inside the clause welche Serie heute läuft, the subject is welche Serie.

If you simplify:

Die Serie läuft heute. → The series is on today.

Here, die Serie is clearly the subject of läuft → nominative.

When you turn it into an indirect question, the structure changes, but the function doesn’t:

welche Serie heute läuft = which series is on today

So welche Serie stays nominative as the subject of läuft.

Why is the verb läuft used for a TV show? Doesn’t laufen mean “to run”?

Literally, laufen does mean “to run” (for people/animals).
But in German, laufen is also commonly used for programs, films, and series:

Was läuft heute im Fernsehen? – What’s on TV today?
Der Film läuft um 20 Uhr. – The film is on at 8 pm.

So in welche Serie heute läuft, läuft means “is on / is airing / is showing”, not “is running” in a physical sense.

Why is heute placed where it is? Could I say welche Serie läuft heute?

In a main clause, you would say:

Welche Serie läuft heute? (direct question)
or
Die Serie läuft heute. (statement)

In the subordinate clause form (welche Serie heute läuft), the conjugated verb must go to the end, so heute stays in front of the verb:

welche Serie heute läuft (indirect question/subordinate clause)

You cannot use main-clause word order inside this subordinate clause.
So welche Serie läuft heute would be incorrect here after the comma.

What’s the difference between Ich frage meine Freundin, welche Serie heute läuft and Ich frage meine Freundin, was heute läuft?

Both are possible, but they focus on different things:

welche Serie heute läuft
→ specifically asking which series is on today.
was heute läuft
→ more general: what is on today (could be news, a film, a show, not necessarily a series).

So:

Use welche Serie, welcher Film, etc. when you know the type and want that detail.
Use was when you’re asking more generally about what’s on.

Could I use ob here instead of welche?

No, ob would change the meaning and doesn’t fit here.

welche Serie = which series (you expect a specific answer: “Series X”)
ob = whether / if (yes/no or alternative question)

Compare:

Ich frage meine Freundin, welche Serie heute läuft.
→ I ask her which series is on today.
Ich frage meine Freundin, ob heute eine Serie läuft.
→ I ask her whether a series is on today (yes or no).

So ob is for yes/no-type questions, welche is for choosing which one.

Does meine Freundin always mean “my girlfriend” (romantic), or can it also mean “my female friend”?

Meine Freundin is ambiguous, but in everyday modern German it most often means:

my girlfriend (romantic partner)

Context can make it clearer. For a non-romantic female friend, speakers often say something like:

eine Freundin von mir – a (female) friend of mine
meine Bekannte – my acquaintance (female), more neutral/formal

So in many contexts, meine Freundin will be understood as “my girlfriend” unless you clarify otherwise.

Why is it Ich frage meine Freundin and not something like Ich bitte meine Freundin here?

Because fragen and bitten are used differently:

fragen (jemanden nach etwas) – to ask (someone) about something / ask a question
- Ich frage sie, welche Serie heute läuft. – I ask her which series is on today.
bitten (jemanden um etwas) – to ask someone for something / request
- Ich bitte sie um Hilfe. – I ask her for help.

In your sentence, you are asking for information, not asking her to do something, so fragen is correct.

Could I say this in the past or future? How would that change the sentence?

Yes, only the main verb tense changes; the subordinate clause stays mostly the same.

Present (now):
Ich frage meine Freundin, welche Serie heute läuft.
Past (I asked):
Ich fragte meine Freundin, welche Serie heute läuft.
(Often people would use the present perfect in spoken German:)
Ich habe meine Freundin gefragt, welche Serie heute läuft.
Future (I will ask):
Ich werde meine Freundin fragen, welche Serie heute läuft.

Note: The verb-at-the-end rule in the subordinate clause remains in all tenses.