Avant de partir, je vais visser la nouvelle poignée.

Because avant needs de before an infinitive.

• avant de + infinitive = before doing something
• So avant de partir means before leaving

Examples:

• Avant de manger, lave-toi les mains. = Before eating, wash your hands.
• Avant de sortir, j’éteins la lumière. = Before going out, I turn off the light.

If a full clause follows, French often uses avant que + subjunctive instead:

• Avant qu’il parte... = Before he leaves...

It is in the infinitive because it follows avant de.

In English, we often use the -ing form after before:

• before leaving

French usually uses:

• avant de partir
• literally: before to leave

So this is a very common French structure:

• après avoir fini = after finishing
• sans parler = without speaking
• avant de commencer = before starting

Je vais visser is the near future (also called futur proche).

It is formed with:

• aller in the present tense + infinitive

So:

• je vais = I am going
• visser = to screw / to fasten with a screw

Together:

• je vais visser = I’m going to screw on / fasten

This is different from the simple future:

• je visserai = I will screw / I’ll screw

Both can refer to the future, but je vais visser often sounds a bit more immediate or planned.

Because je visse is the present tense:

• je visse = I screw / I am screwing

But the sentence means the action will happen soon, after another action:

• Before leaving, I’m going to fasten the new handle.

So the near future is more natural here:

• je vais visser

Compare:

• Je visse la poignée. = I am fastening the handle. / I fasten the handle.
• Je vais visser la poignée. = I’m going to fasten the handle.

Visser means to screw, to fasten with a screw, or sometimes to screw on depending on context.

In this sentence:

• visser la nouvelle poignée means fastening the new handle using a screw or by screwing it into place.

Related words:

• une vis = a screw
• un tournevis = a screwdriver

Be careful: in English, to screw can have informal meanings, but in French visser here is simply the normal practical verb for attaching something with screws.

Because poignée is a feminine noun:

• une poignée
• therefore: la nouvelle poignée

The adjective must agree with the noun in gender and number:

• nouveau → masculine singular
• nouvelle → feminine singular
• nouveaux → masculine plural
• nouvelles → feminine plural

So:

• le nouveau bouton = the new button
• la nouvelle poignée = the new handle

Une poignée usually means a handle.

Depending on context, it can refer to:

• a door handle
• a handle on furniture or a tool
• sometimes a grip or handful in other contexts

Here, because of visser, it most likely means a physical handle being attached, such as:

• a door handle
• a cabinet handle
• a drawer handle

So the sentence suggests installing or fastening a new handle.

Many French adjectives come after the noun, but some common ones often come before it. Nouveau / nouvelle is one of those.

So:

• la nouvelle poignée = the new handle

This is normal French word order.

Other common adjectives often placed before the noun include:

• petit = small
• grand = big/tall
• jeune = young
• vieux = old
• beau = beautiful

Example:

• une petite maison = a small house

Yes, it can.

You can say:

• Avant de partir, je vais visser la nouvelle poignée.
• Je vais visser la nouvelle poignée avant de partir.

Both are correct.

The version with avant de partir at the beginning gives that time idea first:

• Before leaving, ...

The version at the end may sound a little more neutral in some contexts:

• I’m going to fasten the new handle before leaving.

The comma is used because the sentence begins with an introductory phrase:

• Avant de partir, ...

In French, as in English, this kind of opening phrase is often followed by a comma.

If the phrase comes later, there is usually no comma:

• Je vais visser la nouvelle poignée avant de partir.

So the comma is about sentence rhythm and clarity, not a special grammar rule unique to this expression.

A careful pronunciation would be approximately:

ah-vahn duh par-teer, zhuh vay vee-say lah noo-vel pwah-nyay

A few useful points:

• avant: the an is nasal
• de often sounds light, almost like duh
• je vais sounds like zhuh vay
• visser ends with the -é sound: vee-say
• poignée is pronounced roughly pwah-nyay

Also note that gn in poignée sounds like ny in canyon.

Sometimes, but not always.

• partir = to leave, to go away, to depart
• quitter = to leave something or someone behind

So:

• partir often focuses on the act of going away
• quitter usually needs a direct object

Examples:

• Je vais partir. = I’m going to leave.
• Je vais quitter la maison. = I’m going to leave the house.

That is why avant de partir works naturally by itself, while avant de quitter usually needs something after it:

• avant de quitter la maison

Possibly, depending on the object.

In everyday English, handle, knob, and similar words can overlap a bit depending on the object. In French:

• poignée usually suggests a handle
• bouton de porte can refer to a doorknob

But in real usage, context matters a lot. If someone is installing a piece of hardware on a door, cabinet, or drawer, poignée is a very natural word.

So the most straightforward translation is handle, but in some situations English might choose knob depending on the shape.