Hon skruvar fast stolen igen, men vi måste köpa fler skruvar imorgon.

Because the two skruvar are from the same root word but have different grammatical roles:

• Hon skruvar fast ... → here skruvar is a verb: is screwing / screws
• ... köpa fler skruvar ... → here skruvar is a noun in the plural: screws

This is very common in Swedish. A word can be both a noun and a verb, and the sentence structure tells you which one it is.

So in this sentence:

• first skruvar = verb
• second skruvar = plural noun

In skruvar fast, the word fast is a particle, not the adjective fast meaning quick.

skruva fast means:

• to screw on
• to fasten with screws
• to attach by screwing

So Hon skruvar fast stolen igen means something like:

• She is fastening the chair again
• She is screwing the chair back on
• She is screwing the chair back together

This is an example of a particle verb, which is very common in Swedish. The particle changes the meaning of the verb.

In Swedish, particle verbs are often written as two words when the verb is conjugated:

• att skruva fast = infinitive
• hon skruvar fast = present tense
• hon skruvade fast = past tense

So skruva fast is the full verb, but in a sentence the verb part changes form and the particle usually stays separate.

Compare:

• Jag vill skruva fast stolen.
• Jag skruvar fast stolen.

That separation is normal.

No. Here stolen means the chair.

It comes from:

• en stol = a chair
• stolen = the chair

So this is the definite singular form of stol.

It only looks like the English word stolen, but it has nothing to do with theft in this sentence.

Because Swedish marks definiteness mostly by adding an ending to the noun.

• en stol = a chair
• stolen = the chair

So instead of putting a separate word like the before the noun, Swedish often adds the definite ending directly to the noun.

That is why the sentence says stolen: it means a specific chair, not just any chair.

Igen means again.

In this sentence:

• Hon skruvar fast stolen igen = She fastens the chair again

Its position is natural here because it modifies the whole action. Swedish word order with adverbs like igen can vary a little depending on emphasis, but this placement is very common and neutral.

So:

• skruvar fast stolen igen = fastens the chair again

Because fler is used for countable nouns, while mer is used for uncountable things.

• fler skruvar = more screws
  because screws can be counted
• mer vatten = more water
  because water is not normally counted as separate units

Since skruvar means screws, a countable plural noun, fler is correct.

Because fler already works like a determiner here.

In English, we also say:

• buy more screws

not usually

• buy some more screws
  unless we want a slightly different nuance

Swedish does the same kind of thing:

• köpa fler skruvar = buy more screws

So no article is needed before fler.

Because måste is a modal verb, and modal verbs in Swedish are followed by the infinitive without att.

So:

• vi måste köpa = we must buy

Compare:

• Jag vill läsa. = I want to read.
• Hon kan komma. = She can come.
• Vi måste köpa fler skruvar. = We must buy more screws.

This is similar to English, where we also say must buy, not must to buy.

Swedish present tense often covers both:

• she screws
• she is screwing

So Hon skruvar fast stolen igen can mean either:

• She fastens the chair again
• She is fastening the chair again

The exact English translation depends on context. Swedish does not require a separate progressive form like English is doing.

Swedish often uses the present tense together with a time word to talk about the future.

So:

• vi måste köpa fler skruvar imorgon

literally looks like present tense, but it naturally means:

• we have to buy more screws tomorrow

The word imorgon makes the future meaning clear, so no special future verb form is necessary.

Not especially. After men (but), Swedish starts a new main clause.

So the second clause is:

• vi måste köpa fler skruvar imorgon

This follows normal main-clause word order:

• subject: vi
• verb: måste
• infinitive: köpa
• object: fler skruvar
• time expression: imorgon

Swedish main clauses usually follow the V2 rule, meaning the finite verb comes in the second position. Here the first position is vi, so the verb måste comes next.

Here are the main forms:

• hon = she
• skruvar → dictionary form skruva = to screw
• fast = particle in skruva fast
• stolen → dictionary form stol = chair
• igen = again
• men = but
• vi = we
• måste → from måste / related to måsta historically, but you usually just learn måste as must / have to
• köpa = to buy
• fler = more for countable nouns
• skruvar → dictionary form skruv = screw
• imorgon = tomorrow

A useful thing to notice is that one sentence can contain both:

• skruva = verb
• skruv = noun

Sometimes skruva fast can feel close to tighten, but in this sentence the more natural idea is fasten/attach with screws.

So the best understanding is probably that she is:

• fixing the chair in place
• reattaching part of it
• screwing it back together

If Swedish wanted to focus clearly on tightening rather than fastening, it might use different wording depending on context.

So yes, there is a little overlap in the general idea of working with screws, but fasten/attach is the safest meaning here.