Learn Hardcore French: Nous ne voulons pas tomber en panne sur l'autoroute. - We do not want to break down on the highway.

With a conjugated verb followed by an infinitive, French places ne…pas around the conjugated verb: Nous ne voulons pas tomber… is correct. You should not say Nous ne voulons tomber pas…. You can place ne pas before the infinitive (e.g., Nous voulons ne pas tomber…) only for special emphasis or in formal/written style; everyday French prefers the first pattern.

What’s the difference between tomber en panne and être en panne?

Tomber en panne = to break down (the moment something fails).
Être en panne = to be broken down/out of order (the resulting state). In your sentence, you’re talking about the event happening, so tomber en panne fits. If you meant “We don’t want to be stuck broken down,” you could say Nous ne voulons pas être en panne.

Why use sur l’autoroute and not à l’autoroute or dans l’autoroute?

French uses sur for being “on” a road/highway: sur l’autoroute = on the highway.

À l’autoroute would mean “at/by the highway” and is not idiomatic here.
Dans l’autoroute (“in the highway”) is wrong in this context.

Why is it l’autoroute and not la autoroute?

It’s elision: la becomes l’ before a vowel sound. Since autoroute starts with a vowel, you get l’autoroute. Also, autoroute is feminine (une autoroute).

Do I need de after vouloir here?

No. Vouloir + infinitive takes no preposition: vouloir tomber. Compare:

Je veux tomber (I want to fall).
Je veux du café (I want some coffee) — here de is part of the partitive with a noun, not with an infinitive.

How is the whole sentence pronounced?

Roughly: [nu nə vu.lɔ̃ pa tɔ̃.be ɑ̃ pan syʁ l‿o.to.ʁut]
Tips:

nous [nu], final -s silent.
voulons [vu.lɔ̃], nasal -on.
tomber [tɔ̃.be], final -r silent.
en panne [ɑ̃ pan], nasal en.
sur [syʁ], French u.
l’autoroute [lo.to.ʁut], stress evenly.

Are there any liaisons to watch for?

No required liaisons here. Notably:

nous ne: no liaison (next word starts with consonant n).
pas tomber: no liaison (next word starts with consonant t).
sur l’autoroute does have the normal link of the mute -e dropping: [syʁ l‿o.to.ʁut].

Can the ne be dropped in casual speech?

Yes, in everyday spoken French the ne often disappears:

Nous voulons pas tomber en panne… (still quite formal with nous)
More commonly: On veut pas tomber en panne sur l’autoroute.

Is tomber en panne only for vehicles? Why is the subject nous?

The idiom applies to vehicles, machines, and devices. Saying Nous ne voulons pas tomber en panne is shorthand for “we (while driving) don’t want to have a breakdown.” French often accepts human subjects with this idiom to mean “our vehicle breaks down on us.” You could also say:

Notre voiture ne doit pas tomber en panne.
Nous ne voulons pas que la voiture tombe en panne.

Can I front the place phrase for emphasis?

Yes: Sur l’autoroute, nous ne voulons pas tomber en panne. Fronting the location is natural in French for emphasis or topic-setting.

What are some related expressions?

Tomber en panne d’essence = to run out of gas.
Tomber en panne de batterie = the battery dies.
Colloquial: tomber en rade (to break down/conk out).

How is vouloir conjugated in the present?

je veux
tu veux
il/elle veut
nous voulons
vous voulez
ils/elles veulent
Your sentence correctly uses nous voulons.

Could I say “We never want to break down on the highway” or “We don’t want to break down anymore”?

“We never want…”: Nous ne voulons jamais tomber en panne sur l’autoroute.
“We don’t want to… anymore”: Nous ne voulons plus tomber en panne sur l’autoroute.

Why is there no article in en panne?

It’s a fixed prepositional expression: en + noun without an article. Like en retard (late), en forme (in shape), en panne means “out of order/broken down.”

Is Nous voulons ne pas tomber… acceptable?

Grammatically yes, but it’s heavy and rare. It puts focused negation on the infinitive (“we want not to X”), often to contrast options (e.g., “We want not to break down but to arrive safely”). In normal speech, stick to Nous ne voulons pas tomber….