Skruen i bordet er løs.

Because -en is the definite singular ending for many common-gender nouns in Danish.

• skrue = screw
• skruen = the screw

So instead of putting a separate word like the in front, Danish often attaches the definite article to the end of the noun.

For the same reason, but bord is a neuter noun.

• bord = table
• bordet = the table

A very common pattern in Danish is:

• common gender: -en
• neuter: -et

So:

• en skrue → skruen
• et bord → bordet

Because in Danish, the definite article is usually added to the noun itself.

Compare:

• English: the screw

• Danish: skruen

• English: the table
• Danish: bordet

This is one of the first big differences English speakers notice in Danish. Danish does also use separate words like den, det, and de, but usually when an adjective comes before the noun:

• skruen = the screw
• den løse skrue = the loose screw

Here i bordet means something like in the table or fixed into the table.

That makes sense for a screw, because a screw is normally inserted into something.

Compare:

• Skruen i bordet = the screw in the table
• Skruen på bordet = the screw on the table

So på bordet would usually mean the screw is just lying on top of the table, not attached to it.

Because the adjective agrees with the noun, and skruen is common gender singular.

With adjectives used after er, the basic pattern is:

• common singular: løs
• neuter singular: løst
• plural: løse

Examples:

• Skruen er løs. = The screw is loose.
• Hjulet er løst. = The wheel is loose.
  (hjul is neuter.)
• Skruerne er løse. = The screws are loose.

So in your sentence, løs is the correct form.

Yes. The whole noun phrase Skruen i bordet is the subject.

It is made of:

• Skruen = the head noun
• i bordet = a prepositional phrase that describes which screw

So the structure is:

• Skruen i bordet = subject
• er = verb
• løs = subject complement / adjective

This matters because Danish main clauses normally put the finite verb in second position, and here the entire subject phrase counts as the first element.

It can be both.

• en skrue = a screw
• at skrue = to screw / to turn

In this sentence, skruen is definitely a noun, because:

• it has the definite ending -en
• it is followed by er løs, which describes the noun

So here it means the screw, not to screw.

There is no exact English equivalent, but ø is a front rounded vowel.

A useful approximation:

• Start with a vowel somewhat like the one in British bird or her
• Then round your lips

So løs is roughly like lurse without the r sound, but that is only an approximation.

If you know other languages, it is similar to:

• German schön
• French peu

Also, the vowel in løs is long.

Yes. Løs has a wider range of meanings, just like loose in English.

Depending on context, it can mean things like:

• loose
• not fixed tightly
• detached
• free

But in Skruen i bordet er løs, the physical meaning is the natural one: the screw is not tight.

You would say:

En skrue i bordet er løs.

Here:

• en skrue = a screw
• i bordet = in the table
• er løs = is loose

So the difference is:

• Skruen i bordet er løs. = the screw in the table is loose
• En skrue i bordet er løs. = a screw in the table is loose