De flesta har förkläden på sig.

Here de flesta means most people or most of them.

Literally, de flesta is a fixed expression meaning most. In this sentence, there is no noun after it, so English usually fills in something like people from the context.

Swedish often lets de flesta stand on its own when the noun is obvious.

So:

• De flesta har förkläden på sig = Most people are wearing aprons
• De flesta barn har förkläden på sig = Most children are wearing aprons

The second version spells out the noun; the first leaves it understood.

Förkläden is the plural of ett förkläde.

The forms are:

• singular indefinite: ett förkläde
• singular definite: förklädet
• plural indefinite: förkläden
• plural definite: förklädena

So in this sentence, förkläden means aprons.

Because the sentence talks about a group of people, Swedish naturally uses the plural for the clothing item across the group.

So De flesta har förkläden på sig means that, taken together, most of them are wearing aprons.

You could also say De flesta har ett förkläde på sig, which highlights one apron each more clearly. But the plural version is very natural when talking about many people at once.

På sig is part of the expression ha ... på sig, which means to wear.

Literally, it is something like have ... on oneself:

• på = on
• sig = oneself, himself, herself, themselves

So:

• ha förkläden på sig = wear aprons

In everyday Swedish, this is one of the most common ways to talk about wearing clothes.

Sig is the reflexive pronoun used for third person subjects in Swedish.

Since the subject is de flesta and that refers to they here, Swedish uses sig:

• De flesta har förkläden på sig

Compare with other subjects:

• Jag har ett förkläde på mig
• Du har ett förkläde på dig
• Vi har förkläden på oss
• Ni har förkläden på er
• De har förkläden på sig

So sig matches a third-person subject.

With clothing, Swedish very often puts the thing being worn before på sig:

• ha en jacka på sig
• ha skor på sig
• ha förkläden på sig

That is a very normal pattern.

You may also hear ha på sig followed by the clothing item in some contexts, but ha + clothing + på sig is extremely common and very natural.

A useful way to think of it is that på sig belongs to the verb phrase, but the clothing noun is often inserted before it.

Because förkläden is an indefinite plural noun.

In Swedish, indefinite plural nouns usually have no article:

• ett förkläde = an apron
• förkläden = aprons

So har förkläden på sig simply means are wearing aprons.

If you meant specific aprons already known in the conversation, you would use the definite form förklädena.

Yes. In everyday Swedish, ha ... på sig is one of the most natural ways to say wear.

There is also bära, which can mean wear, but it is often a bit more formal, descriptive, or stylistic.

For everyday clothing sentences, Swedish very often prefers:

• Hon har en klänning på sig
• Han har hatt på sig
• De flesta har förkläden på sig

So if you want to sound natural in ordinary conversation, ha ... på sig is a great pattern to learn.

Yes:

• De flesta har ett förkläde på sig

That version is also grammatical. It focuses a bit more on the idea that each person has one apron.

The original plural version, De flesta har förkläden på sig, is still very natural because the sentence is talking about many people as a group.