Nous avons dû dévisser la poignée avant de visser la nouvelle porte du placard.

Nous avons dû is the passé composé of devoir and means a completed necessity in the past: we had to.

In this sentence, it refers to one specific action sequence that happened and was completed.

By contrast, nous devions is the imparfait and usually suggests:

• an ongoing obligation,
• background information,
• or something that was supposed to happen.

So:

• Nous avons dû dévisser la poignée... = we had to unscrew the handle.
• Nous devions dévisser la poignée... = we were supposed to / had to unscrew the handle, with a less completed, more background-like feel.

Because devoir works like a modal verb here, similar to English to have to.

After verbs like:

• devoir = must / have to
• pouvoir = can / to be able to
• vouloir = want to

the next verb usually stays in the infinitive.

So:

• nous avons dû dévisser = we had to unscrew
• il peut venir = he can come
• je veux partir = I want to leave

Only devoir is conjugated here; dévisser stays in its base form.

Dû is the past participle of devoir in the masculine singular form.

The circumflex is the standard traditional spelling and also helps distinguish it from du, which usually means of the or some as a contraction of de + le.

Compare:

• J’ai dû partir. = I had to leave.
• la porte du placard = the cupboard door / the door of the cupboard

Related forms are:

• dû
• due
• dus
• dues

Only the masculine singular form normally has the circumflex.

Visser means to screw in, to fasten with a screw, or more generally to attach by screwing.

Dévisser means to unscrew or to remove by turning a screw out.

The prefix dé- often gives the idea of:

• undoing,
• reversing,
• removing.

So here:

• dévisser la poignée = remove the handle by unscrewing it
• visser la nouvelle porte = fasten/install the new door with screws

French usually uses avant de + infinitive when the subject is the same for both actions.

Here, we did both things:

1. unscrewed the handle,
2. then screwed on / installed the new cupboard door.

So:

• avant de visser = before screwing / before installing

If the subject changes, French normally uses avant que + subjunctive:

• Nous avons attendu avant qu’il ne visse la porte.
  = We waited before he screwed in the door.

So the rule is basically:

• same subject → avant de + infinitive
• different subject → avant que + subjunctive

Du is the contraction of de + le.

Since placard is a masculine singular noun, de le placard becomes du placard.

So:

• la porte du placard literally = the door of the cupboard
• natural English = the cupboard door or sometimes the closet door

Other patterns:

• de la for feminine singular
• de l’ before a vowel sound
• des for plural

Examples:

• la porte de la cuisine
• la clé de l’armoire
• les portes des placards

Because nouveau / nouvelle is one of the adjectives that usually comes before the noun in French.

So:

• la nouvelle porte = the new door

Many common adjectives of size, age, beauty, and goodness often come before the noun. Learners sometimes remember this through the BAGS idea:

• Beauty
• Age
• Goodness
• Size

Nouveau falls into the age/newness group.

If you put an adjective after the noun, it can sometimes change the tone or meaning, but la nouvelle porte is the normal everyday order here.

La is used because the speaker is talking about a specific door, not just any new door.

So:

• la nouvelle porte du placard = the specific new cupboard door
• une nouvelle porte de placard = a new cupboard door, one among others

French articles often depend on whether the noun is:

• specific and identifiable → le / la / les
• non-specific → un / une / des

In this sentence, the context makes the door specific.

It can mean either, depending on context.

Placard usually refers to a built-in storage space, cabinet, or cupboard. In some contexts, English speakers might translate it as:

• cupboard
• cabinet
• closet

So la porte du placard could be:

• the cupboard door
• the cabinet door
• the closet door

The exact English choice depends on what kind of furniture or storage space is being described.

Poignée is pronounced roughly like pwah-nyay.

A few useful points:

• poi sounds like pwa
• gn in French sounds like ny, similar to the sound in English canyon
• the final ée sounds like ay

So:

• poignée ≈ pwa-nyay

Also useful in this sentence:

• nous avons has a liaison: nou-z-avon
• placard ends with a silent d

Not necessarily in an awkward literal sense. In French, visser can mean to fasten or attach something using screws.

So visser la nouvelle porte naturally means something like:

• screw on the new door
• attach the new door with screws
• install the new door

French often uses a direct verb like this where English might choose a slightly broader verb such as install or attach.