Learn Hardcore Dutch: De tram stopte ineens, maar iedereen bleef rustig. - The tram stopped suddenly, but everyone stayed calm.

The stem of stoppen is stop. For weak verbs, you add -te when the stem ends in a voiceless consonant (’t kofschip/XTC-koffie rule). Since p is voiceless, you get stopte.
You don’t double the t: the -te supplies the single t.
stopde would follow a voiced consonant stem; that’s not the case here.

Can I say De tram is ineens gestopt instead of De tram stopte ineens?

Yes. Both are correct. The simple past (stopte) is common in narratives; the present perfect (is gestopt) is very common in speech for completed events. Note that intransitive stoppen takes zijn: De tram is gestopt.

Why iedereen bleef and not iedereen bleven?

Iedereen is grammatically singular, so the verb is singular: bleef. For a plural verb, use a plural subject: Ze bleven allemaal rustig.

Is bleef irregular? What are the other forms of blijven?

Yes, blijven is a strong verb.

Present: ik blijf, hij blijft
Simple past: ik/hij bleef, wij bleven
Past participle: gebleven (with zijn: ik ben gebleven)

Why is it rustig and not rustige after bleef?

Blijven is a linking verb (like zijn and worden), so it takes a predicative adjective without -e: hij bleef rustig. You add -e only before a noun: een rustige passagier.

Is rustig an adjective or an adverb here?

Here it’s a predicative adjective describing the subject’s state. As an adverb it can mean “quietly” (Hij praat rustig). With blijven, think “remain + adjective.”

Can ineens go elsewhere? For example, Ineens stopte de tram?

Yes. Dutch main clauses are verb-second. If you front Ineens, the finite verb comes next and the subject follows: Ineens stopte de tram, maar iedereen bleef rustig. Keeping ineens after the verb (De tram stopte ineens) is neutral.

What’s the difference between ineens, opeens, plotseling, and plots?

All mean “suddenly,” with register/region nuances:

ineens: neutral, very common.
opeens: also common; slightly more colloquial/regional.
plotseling: a bit more formal/written.
plots: common in Belgium; in the Netherlands it can sound Belgian/poetic.

Is the comma before maar required?

It’s optional with coordinating conjunctions (en, maar, of). Many writers include it to signal the pause between two main clauses; your comma is fine.

Does maar change the word order in the second clause?

No. Maar is coordinating, so the second clause keeps regular main-clause order (Subject–Verb–…): iedereen bleef rustig. No inversion is triggered by maar itself.

Are there other natural ways to say this?

Yes, with slight nuance shifts:

De tram kwam plotseling tot stilstand (more formal; result-focused).
De tram stond plots stil (focus on the state of being stationary).
If an agent causes it: De bestuurder zette de tram abrupt stil.

Is halte related to stopte? I’ve seen tramhalte.

They’re different:

stopte = past of stoppen (“stopped”).
(tram)halte = a noun meaning “(tram) stop/station.” Example: De tram stopte bij de halte.

Can I use stoppen met here, like stoppen met roken?

Stoppen met means “to quit doing something”: Hij stopte met roken. For vehicles, you usually just say De tram stopte. If you add met, you need an activity: De tram stopte met rijden (grammatical but rarely said).

Pronunciation tips for the tricky bits?

stopte: [ˈstɔp.tə] (short o; crisp p+t; final -e is a schwa)
ineens: [ɪˈneːns] (long ee)
bleef: [bleːf] (long ee)
rustig: [ˈrʏs.tɪx] in NL Dutch (final g like German “Bach”)
maar: [maːr]
iedereen: [ˌidəˈreːn]

Does maar ever mean “only”? Any risk of confusion here?

Yes, maar can mean “only” in contexts like maar tien euro (“only ten euros”). In your sentence it’s clearly the conjunction “but,” so there’s no ambiguity.

Could I use werd instead of bleef?

Different meaning:

bleef rustig = “remained calm” (they were already calm).
werd rustig = “became calm” (they calmed down after being agitated).

Where would you put a place phrase like bij de halte?

Natural options:

De tram stopte ineens bij de halte, maar iedereen bleef rustig.
Bij de halte stopte de tram ineens, maar iedereen bleef rustig. (fronted place; verb-second inversion)