Learn Hardcore French: Nous devons agir ensemble.

With devoir, you use an infinitive to express obligation or necessity: devoir + infinitif. So nous devons agir literally means we have the obligation to act.

Can I say On doit agir ensemble instead of Nous devons agir ensemble?

Yes. On is very common in spoken French and often replaces nous. Meaning is the same, but:

Nous devons is a bit more formal or written.
On doit sounds more conversational. Grammar changes to 3rd person singular: on doit, not on devons.

How do you pronounce the whole sentence?

IPA: /nu dəvɔ̃ zaʒiʁ ɑ̃sɑ̃bl/

Tips:

devons: /dəvɔ̃/ (nasal vowel at the end).
Liaison between devons and agir gives a /z/ sound: /dəvɔ̃z‿aʒiʁ/.
agir: the g before i is the zh sound /ʒ/.
ensemble: /ɑ̃sɑ̃bl/ (two nasal vowels; final -e is silent).

Are there any liaisons I should make?

Yes: between devons and agir → /dəvɔ̃z‿aʒiʁ/.
No liaison after nous here because devons starts with a consonant (so not /nuz dəvɔ̃/, but /nu dəvɔ̃/).

Is ensemble variable? Do I ever add an s?

As an adverb meaning together, ensemble is invariable—never add an s.
Only the noun un ensemble (a set/outfit) takes a plural (des ensembles), which is a different use.

Where can ensemble go in the sentence?

Most natural: at the end: Nous devons agir ensemble.
Fronted for emphasis: Ensemble, nous devons agir.
To intensify: Nous devons agir tous ensemble. Avoid the pleonasm ensemble avec X; prefer either avec X or ensemble:
Nous devons agir avec eux. / Nous devons agir ensemble.

What’s the difference between agir and faire here?

Agir = to take action, to act (in a general or abstract sense).
Faire = to do/make something specific.
Here, French prefers agir for the idea of taking action; faire would sound off unless you specify an object (e.g., faire quelque chose).

How do I make it negative, and does that mean “don’t have to”?

Prohibition: Nous ne devons pas agir ensemble. = we must not act together.
Lack of necessity (“don’t have to”): avoid ne pas devoir; use:
- Nous n’avons pas besoin d’agir ensemble.
- Nous ne sommes pas obligés d’agir ensemble.

How do I soften “must” to “should”?

Use the conditional of devoir: Nous devrions agir ensemble. This expresses advice or recommendation rather than strict obligation.

How do I say it in other tenses?

Future: Nous devrons agir ensemble.
Imparfait (ongoing/past background): Nous devions agir ensemble.
Passé composé (completed necessity): Nous avons dû agir ensemble.

Does devoir also mean “to owe”?

Yes. Devoir can mean to owe (money, favors): Nous devons 20 €. Context tells you which meaning applies. With an infinitive (devoir + infinitif), it’s obligation/probability; with a noun or amount, it’s owing.

Can I express the same idea with il faut?

Yes:

General necessity: Il faut agir ensemble.
With a specific subject (requires the subjunctive): Il faut que nous agissions ensemble.

How do I say “Let’s act together!”?

Use the 1st person plural imperative of agir: Agissons ensemble !

What’s the difference between agir and s’agir de?

Agir = to act/take action.
S’agir de is an impersonal expression used only with il: Il s’agit de… = it’s about / it concerns.
You cannot say nous nous agissons in this sense; that’s ungrammatical.

Do I need tous before ensemble?

Not required, but it changes nuance:

Nous devons agir ensemble. = we must act together (as a group).
Nous devons tous agir ensemble. = every one of us must act, and do so together (emphasizes each individual).
Nous devons agir tous ensemble. = stresses the idea of acting all at the same time.

How do I turn it into a yes/no question?

Three common ways:

Colloquial intonation: Nous devons agir ensemble ?
With est-ce que: Est-ce que nous devons agir ensemble ?
Inversion (more formal): Devons-nous agir ensemble ?