Sá sem kemur seint þarf að bíða lengur.

Sá sem is a very common Icelandic pattern meaning the one who, he who, or more naturally in English, whoever.

• sá = that one / he
• sem = a relative word like who / that

So Sá sem kemur seint literally means the one who comes late.

In sentences like this, sá sem is often used in a general sense, not necessarily about one specific male person. It is a standard way to introduce a rule or general statement.

Kemur is the 3rd person singular present form of koma (to come).

The subject is Sá sem kemur seint = the one who comes late, which is treated as singular, so the verb must also be singular:

• ég kem = I come
• þú kemur = you come
• hann/hún/það kemur = he/she/it comes

Here, the person is not named directly, but sá functions like a singular subject, so kemur is the correct form.

Because the subject is singular.

The whole subject is:

Sá sem kemur seint = the one who comes late

That whole phrase counts as one person, so the verb is singular:

• þarf = needs / has to (3rd person singular)
• þurfa = need / have to (plural, or infinitive depending on context)

Compare:

• Sá sem kemur seint þarf að bíða lengur. = The one who comes late has to wait longer.
• Þeir sem koma seint þurfa að bíða lengur. = Those who come late have to wait longer.

Because þurfa is normally followed by að + infinitive.

So:

• þurfa að bíða = to have to wait
• þarf að fara = has to go
• þarf að lesa = has to read

Here:

• þarf = has to
• að bíða = to wait

This is a very common structure in Icelandic, and it works much like English have to + verb, except Icelandic usually inserts að before the infinitive.

Seint is an adverb, meaning late.

It is not describing a noun; it is describing how someone comes, so Icelandic uses an adverb:

• koma seint = come late

This is similar to English, where late can function adverbially too.

Related forms:

• seinn = late (adjective, masculine form)
• seint = late (adverb, and also neuter adjective form in other contexts)

In this sentence, seint is definitely adverbial because it modifies the verb kemur.

Lengur means longer. It is the comparative adverb.

• lengi = for a long time / long
• lengur = longer

So:

• bíða lengi = wait a long time
• bíða lengur = wait longer

The sentence needs the comparative idea longer, not just for a long time, so lengur is the right choice.

Because the whole phrase Sá sem kemur seint is the subject, and then the main verb þarf comes after it.

Icelandic main clauses usually follow a verb-second pattern. That means the finite verb tends to come in the second position, but the first position can be an entire phrase, not just one word.

So the structure is:

1. Sá sem kemur seint = the full subject phrase
2. þarf = the finite verb
3. að bíða lengur = rest of the predicate

So even though several words come before þarf, they all belong to one unit: the subject.

Often yes, but not always in a one-to-one way.

In this sentence, sem works like English who in the one who comes late.

However, sem is a very general relative particle in Icelandic and can also correspond to English that, which, or sometimes be left untranslated depending on the sentence.

So here:

• Sá sem kemur seint = the one who comes late

But it is better to think of sem as Icelandic’s normal relative linker, rather than trying to force it to match only one English word.

Yes. Bíða often takes a genitive object when you are waiting for someone or something.

For example:

• að bíða strætósins = to wait for the bus
• að bíða einhvers = to wait for someone/something

But in your sentence, there is no explicit object after bíða. It just means wait, so no object appears:

• þarf að bíða lengur = has to wait longer

That is why you do not see any case marking here, even though bíða often teaches learners to expect the genitive.

Yes, absolutely.

That is one of the most natural English ways to express the idea. Even though sá sem is literally the one who, English often prefers whoever for a general rule like this.

So these are both good ways to understand it:

• The one who comes late has to wait longer.
• Whoever comes late has to wait longer.

The Icelandic sentence itself is very natural for making that kind of general statement.