Nous avons longtemps attendu dans la salle d'attente.

Avons attendu is the passé composé, one of the most common ways to talk about a completed action in the past in French.

It is made with:

• the auxiliary verb avoir
• plus the past participle attendu

So:

• nous avons attendu = we waited / we have waited

In everyday French, the passé composé is usually used where English often uses the simple past:

• Nous avons attendu = We waited

French does have a simple past form called the passé simple, but you normally see that in literature, not ordinary conversation.

Most French verbs form the passé composé with avoir, and attendre is one of them.

So:

• attendre → avoir attendu
• nous avons attendu

Only certain verbs use être, especially many verbs of movement or change of state, such as:

• aller → nous sommes allés
• venir → nous sommes venus

Because attendre is not one of those être verbs, it uses avoir.

In the passé composé with avoir, the past participle usually does not agree with the subject.

So even though nous means we, the verb stays:

• nous avons attendu

Not:

• nous avons attendus

Agreement with avoir only happens in a special case: when a direct object comes before the past participle.

For example:

• Les personnes que nous avons attendues
  • Here attendues agrees with les personnes because that direct object comes before the verb.

But in your sentence, there is no preceding direct object, so it stays attendu.

Longtemps means for a long time or a long while.

In compound tenses like the passé composé, adverbs of time or duration often go between the auxiliary and the past participle:

• Nous avons longtemps attendu

That placement is very natural in French.

You may also see:

• Nous avons attendu longtemps

That is also correct and natural.
The version with longtemps between avons and attendu can sound a little more polished or rhythmically smooth, but both are fine.

This is a very common learner question.

• longtemps = for a long time
• pendant longtemps = also for a long time, but often with more emphasis on duration as a stretch of time

In many sentences, they can be quite similar:

• Nous avons longtemps attendu
• Nous avons attendu pendant longtemps

Both are understandable and correct.

However, longtemps is often the more natural, compact choice in everyday French. It is extremely common with verbs like attendre, vivre, travailler, espérer, etc.

Yes. Attendre can be used on its own, just like to wait in English.

So:

• Nous avons attendu = We waited

You can also add what you were waiting for:

• Nous avons attendu le bus = We waited for the bus
• J’attends Marie = I’m waiting for Marie

A very important point for English speakers: French attendre usually takes its object directly, without for.

So:

• attendre le bus
• attendre un ami

Not:

• attendre pour le bus
• attendre pour un ami

Dans means in or inside, so dans la salle d’attente means in the waiting room.

This is a normal way to express location:

• dans la maison = in the house
• dans la voiture = in the car
• dans la salle d’attente = in the waiting room

French uses dans because you are physically inside that space.

Literally, salle d’attente is room of waiting, but in natural English it means waiting room.

It is a very common French noun pattern:

• salle de bain = bathroom
• salle à manger = dining room
• salle d’attente = waiting room

Here, d’ is just de shortened before a vowel:

• de + attente → d’attente

So la salle d’attente is a fixed expression meaning the waiting room.

The apostrophe shows elision, which means a vowel is dropped before another vowel sound.

• de attente would sound awkward
• so French changes it to d’attente

This happens very often in French:

• le ami → l’ami
• je aime → j’aime
• de école → d’école

So:

• salle d’attente not
• salle de attente

Yes, absolutely.

In everyday spoken French, on is often used instead of nous:

• On a longtemps attendu dans la salle d’attente

This still means we waited a long time in the waiting room.

The difference is mainly register:

• nous avons... = a bit more formal or careful
• on a... = very common in conversation

Both are correct.

A careful pronunciation would be roughly:

Noo-zavon lon-tan ah-tahn-doo dahn lah sal dah-tahnt

A few useful pronunciation notes:

• Nous avons has a liaison: the s in nous links to avons, so it sounds like noo-zavon
• longtemps has a nasal vowel; the final ps is silent
• attendu begins with a, and the tt is not pronounced like a strong English t
• dans also has a nasal vowel
• salle d’attente flows together smoothly because of the apostrophe in d’attente

Not with the same meaning.

Nous avons longtemps attendu presents the waiting as a completed past event.

Nous attendions is the imparfait, which is used more for:

• background description
• ongoing past actions
• habitual past actions

So:

• Nous avons longtemps attendu dans la salle d’attente = We waited a long time in the waiting room.
• Nous attendions dans la salle d’attente quand... = We were waiting in the waiting room when...

The passé composé is the better choice if you are simply reporting that the waiting happened and was completed.