Paul essaie de visser la planche, mais il doit d’abord dévisser l’ancienne vis.

Because essayer is commonly followed by de + infinitive when it means to try to do something.

• essayer de visser = to try to screw
• essayer de comprendre = to try to understand

In everyday French, you may also hear essayer à + infinitive in some contexts, but essayer de + infinitive is the standard form learners should use first.

So:

• Paul essaie de visser... = Paul is trying to screw...

These two verbs are opposites:

• visser = to screw in / fasten with a screw
• dévisser = to unscrew / remove by turning

The prefix dé- often gives the idea of undoing, reversing, or removing an action.

Examples:

• visser une vis = to screw in a screw
• dévisser une vis = to unscrew a screw

So in this sentence, Paul wants to fasten the board, but first he has to remove the old screw.

French uses articles much more often than English does. Where English might say screw the board or remove an old screw, French often prefers a clear article before the noun.

Here:

• la planche = the board
• l’ancienne vis = the old screw

A few notes:

• la is used before a feminine singular noun.
• l’ is used instead of la before a vowel sound, as in ancienne.

So:

• la vis
• but l’ancienne vis

Because ancien/ancienne usually comes before the noun when it means old/former in a common descriptive way.

So:

• une ancienne vis = an old screw

Many common adjectives in French come before the noun, especially short and frequent ones. Ancien/ancienne is one of them in many cases.

Also, adjective position can sometimes affect meaning:

• une ancienne vis = an old/former screw
• une vis ancienne could sound more like an antique/very old screw, depending on context

For normal everyday French in this sentence, l’ancienne vis is the natural choice.

Because the subject is il (he), so the verb devoir must be conjugated for il.

Present tense of devoir:

• je dois = I must / I have to
• tu dois = you must
• il/elle doit = he/she must
• nous devons
• vous devez
• ils/elles doivent

Since the sentence says il doit, it matches Paul.

D’abord means first or first of all.

In this sentence:

• il doit d’abord dévisser... = he must first unscrew...

It is placed before the infinitive phrase to show the order of actions:

1. remove the old screw
2. then screw the board

French word order here is very natural:

• il doit d’abord dévisser...

You could also hear similar time-order words like:

• ensuite = then
• puis = then
• avant de... = before doing...

Because both are dependent on another verb.

• essaie de visser: after essaie de, the action verb stays in the infinitive
• doit dévisser: after doit, the next verb also stays in the infinitive

This is very common in French.

Pattern:

• conjugated verb + infinitive

Examples:

• Je veux partir. = I want to leave.
• Elle essaie de comprendre. = She is trying to understand.
• Nous devons attendre. = We must wait.

So only essaie and doit are conjugated here; visser and dévisser remain infinitives.

Yes, it can look odd at first. essaie is pronounced roughly like eh-say.

A few pronunciation points:

• essaie = /e.sɛ/ approximately
• the final -e is not pronounced
• the ai here sounds like è / eh

Similarly:

• visser sounds roughly like vee-say
• dévisser sounds roughly like day-vee-say

And:

• doit sounds roughly like dwah
• vis sounds like vees

They are related words:

• une vis = a screw (noun)
• visser = to screw / to fasten with a screw (verb)

French often forms verbs from nouns this way. Here, the connection is very direct:

• vis = the object
• visser = the action done with that object

Likewise:

• dévisser = to remove by unscrewing

So in the sentence:

• l’ancienne vis = the old screw
• visser la planche = to screw the board in place / fasten the board with a screw

Grammatically, visser la planche means he is fastening the board using a screw. French often states the object being attached rather than spelling out the entire setup.

So the sentence does not necessarily mean he is literally screwing into the board only; it means he is working on fastening the board, and to do that he first has to remove the old screw.

This is a normal, natural way to say it in French, even if English might sometimes express the physical details differently.

Yes, mais here is simply but.

It connects two contrasting ideas:

• Paul essaie de visser la planche = Paul is trying to screw the board
• mais il doit d’abord dévisser l’ancienne vis = but he must first unscrew the old screw

So mais works very much like English but in this sentence.