Nous allons passer une soirée formidable ensemble.

Nous allons passer is the near future in French: aller + infinitive.

• nous allons passer = we are going to spend / we’re going to have
• nous passerons = we will spend

Both can work, but nous allons passer often sounds a bit more immediate or natural in everyday speech, especially when talking about a plan or expectation.

So this sentence feels like:

• We’re going to have a wonderful evening together

rather than a more neutral or formal:

• We will have a wonderful evening together

Here, passer means to spend in the sense of spending time.

So:

• passer une soirée = to spend an evening
• more naturally in English, often to have an evening

Examples:

• passer une journée = to spend a day
• passer de bonnes vacances = to have/spend a good vacation

So even though passer often means to pass, in this context it means to spend time.

French often distinguishes between:

• un soir = an evening / one evening / in the evening, often referring to a point in time
• une soirée = the evening as an event, experience, or period of time spent doing something

In this sentence, the idea is having/spending an enjoyable evening, so une soirée is the natural choice.

Compare:

• Un soir, je l’ai vu au café. = One evening, I saw him at the café.
• Nous avons passé une belle soirée. = We had a lovely evening.

French normally uses an article where English sometimes does too, but the structure matters.

Here, une soirée means an evening. The noun needs an article:

• passer une soirée
• passer une bonne journée
• passer un bon moment

You would not normally say passer soirée in standard French.

Not usually. This is a very common trap.

In French, formidable usually means:

• great
• wonderful
• fantastic
• terrific

In English, formidable often means:

• impressive and powerful
• intimidating

So in this sentence, une soirée formidable means:

• a wonderful evening not
• a formidable evening in the usual English sense

Ensemble means together.

In this sentence, it comes at the end:

• Nous allons passer une soirée formidable ensemble.

That placement is very natural in French. It tells you that we will spend the evening together.

French often puts words like ensemble after the main idea:

• Nous irons au cinéma ensemble.
• Ils travaillent bien ensemble.

Sometimes yes, but the original position is the most natural here.

Most natural:

• Nous allons passer une soirée formidable ensemble.

Possible but less neutral:

• Nous allons ensemble passer une soirée formidable.

That version sounds more marked or literary. For everyday French, putting ensemble at the end is usually best.

Because allons is the nous form of the verb aller.

Present tense of aller:

• je vais
• tu vas
• il / elle / on va
• nous allons
• vous allez
• ils / elles vont

So:

• nous allons passer = we are going to spend

In everyday spoken French, many people would more naturally say:

• On va passer une soirée formidable ensemble.

This means the same thing as:

• Nous allons passer une soirée formidable ensemble.

Difference:

• nous is a bit more formal, careful, or written
• on is very common in everyday speech for we

So the original sentence is completely correct, but in conversation you may hear on va more often than nous allons.

Because ensemble itself already functions as an adverb meaning together.

So French simply says:

• vivre ensemble = live together
• travailler ensemble = work together
• sortir ensemble = go out together
• passer une soirée ensemble = spend an evening together

No extra word like with is needed before it.

A careful pronunciation would be approximately:

Noo zah-lon pah-say oon swah-ray for-mee-dab-l ahn-sombl

A few useful points:

• Nous allons has a liaison, so the s in nous connects to allons:
  • nou-z-allons
• une soirée: soirée sounds roughly like swah-ray
• formidable in French is pronounced very differently from English:
  • roughly for-mee-dabl
• ensemble sounds like ahn-sombl

You do not pronounce every final letter strongly the way you might expect from English spelling.

Normally, no. In this sentence, the natural order is:

• une soirée formidable

Most French adjectives come after the noun, and formidable is one of them.

So:

• une soirée formidable = correct
• une formidable soirée = possible, but more literary or emphatic

The version with the adjective after the noun is the standard, neutral one.

Not in normal English translation. Even though passer can sometimes literally mean to pass, here passer une soirée is the standard expression for spending an evening.

So the natural meaning is:

• We’re going to spend a wonderful evening together or
• We’re going to have a wonderful evening together

That is how a native speaker would understand it.

Yes, it is completely natural and correct.

It sounds like something you might say:

• when making plans
• when reassuring someone
• when expressing excitement about an evening together

A very everyday spoken version would be:

• On va passer une soirée formidable ensemble.

But the original sentence is perfectly good French.