Si tu veux visser la nouvelle poignée, prends le tournevis qui est dans la boîte à outils.

Here si means if.

So:

Si tu veux visser la nouvelle poignée... = If you want to screw on the new handle...

French si can also mean yes, but only in a different situation: when answering a negative question or statement.

• Tu n’as pas faim ? — Si ! = Aren’t you hungry? — Yes, I am!

That is not what is happening in this sentence.

Because after vouloir (to want), French usually uses an infinitive, just like English often uses want to + verb.

• tu veux visser = you want to screw
• je veux partir = I want to leave
• nous voulons apprendre = we want to learn

So Si tu veux visser... is a very normal structure.

Because this is the imperative: a direct command.

• tu prends = you take / you are taking
• prends ! = take!

French imperative often drops the subject pronoun:

• Prends le tournevis. = Take the screwdriver.
• Regarde. = Look.
• Écoutez. = Listen.

So prends is the correct command form for speaking to one person informally.

Because prendre is not an -er verb.

In the tu imperative:

• many -er verbs lose the final -s
  
  • regarde
  • ferme
• but verbs like prendre, finir, mettre, etc. keep it
  
  • prends
  • finis
  • mets

So prends is the normal imperative form of prendre.

That is a very common question. It is true that many French adjectives come after the noun, but some common adjectives often come before it.

Nouveau / nouvelle is one of those adjectives, so:

• la nouvelle poignée = the new handle

This is the normal order.

Compare:

• une nouvelle voiture = a new car
• un nouveau problème = a new problem

If you put nouvelle after the noun, it can sound unusual, literary, or change the emphasis.

Un tournevis means a screwdriver.

It is a compound word built from:

• tourner = to turn
• vis = screw

So the literal idea is something like turn-screw.

A useful detail: tournevis is usually written as one word.

Because qui is the subject of est in this relative clause.

• le tournevis qui est dans la boîte à outils
  = the screwdriver that is in the toolbox

Here qui refers to le tournevis, and qui is doing the job of that/which as the subject of is.

A quick comparison:

• le tournevis qui est sur la table = the screwdriver that is on the table
  (qui = subject)
• le tournevis que je cherche = the screwdriver that I’m looking for
  (que = object)

So qui est is correct here.

In boîte à outils, the à shows purpose or function: a box for tools.

So:

• une boîte à outils = a toolbox
• literally: a box for tools

This pattern is common in French:

• une tasse à café = a coffee cup
• une machine à laver = a washing machine
• une brosse à dents = a toothbrush

So à here does not mean to in the usual movement sense. It helps build a noun phrase meaning for / used for.

Because the sentence is addressing one person informally.

• tu = singular, informal
• vous = plural, or singular formal

So this sentence sounds like something said to a friend, family member, child, or someone you are on tu terms with.

The formal version would be:

Si vous voulez visser la nouvelle poignée, prenez le tournevis qui est dans la boîte à outils.

It is literal, but it can also make the instruction sound a little less abrupt.

Compare:

• Prends le tournevis. = Take the screwdriver.
• Si tu veux visser la nouvelle poignée, prends le tournevis...
  = If you want to screw on the new handle, take the screwdriver...

The second version sounds more like practical guidance: if that is what you’re trying to do, this is what you need.

So yes, it is literal, but it can also make the command feel more natural and less blunt.

Yes, a few:

• poignée is pronounced roughly pwa-nyay
• tournevis is roughly toor-nuh-vee
• boîte sounds roughly like bwat
• outils sounds roughly like oo-tee

A few specific things to notice:

• gn in poignée is like the ny sound in canyon
• the final -s in outils is silent
• tu in French is not the same vowel as English too; it is more rounded

A rough pronunciation of the whole sentence could be:

See tyu vuh vee-say la noo-vel pwa-nyay, prahn luh toor-nuh-vee kee eh dahn la bwat ah oo-tee.

That is only an approximation, but it can help at the beginning.