Ef þú kemur seint, þá bíð ég í smástund.

Icelandic very often uses the present tense to talk about the future when the meaning is clear from context.

So:

• Ef þú kemur seint = literally If you come late
• þá bíð ég í smástund = literally then I wait a little while

But in natural English, this is often translated as:

• If you come late, then I’ll wait a little while

This is normal in Icelandic, especially in if-clauses, schedules, plans, and other situations where the future meaning is obvious.

Kemur is the 2nd person singular present tense of koma (to come).

The subject is þú (you, singular), so the verb has to match that:

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

So Ef þú kemur seint literally means If you come late.

Bíð is the 1st person singular present tense of bíða (to wait).

Because the subject is ég (I), Icelandic uses:

• ég bíð = I wait
• þú bíður = you wait
• hann/hún/það bíður = he/she/it waits

So bíð ég means I wait, just with a different word order than English.

This is because Icelandic follows a verb-second pattern in main clauses.

In this sentence, the first part is:

• Ef þú kemur seint = If you come late

That whole clause comes first. After that, the main clause begins, and the finite verb usually comes early, before the subject:

• Ef þú kemur seint, þá bíð ég í smástund.

So the order is not random. Icelandic is not simply copying English word order.

A useful way to think of it:

1. first element: Ef þú kemur seint
2. then in the main clause: finite verb bíð
3. then subject ég

You will see this kind of inversion a lot after fronted phrases or clauses.

Þá here means something like then.

In a sentence like this, it helps connect the result clause to the if-clause:

• Ef þú kemur seint, þá bíð ég í smástund.
• If you come late, then I’ll wait a little while.

It is often natural, but not always strictly necessary. You can also see:

• Ef þú kemur seint, bíð ég í smástund.

That still makes sense. Adding þá just makes the connection a little clearer or more explicit.

In sentences like this, yes: ef is the normal word for if.

Examples:

• Ef ég hef tíma, kem ég. = If I have time, I’ll come.
• Ef það rignir, verðum við heima. = If it rains, we’ll stay home.

So in your sentence, ef introduces the condition.

Seint is an adverb meaning late.

It describes how someone comes:

• koma seint = to come late

It is not an adjective here. It does not describe a noun; it describes the action of the verb.

Compare:

• Hann er seinn. = He is late.
  Here seinn is an adjective.
• Hann kemur seint. = He comes late.
  Here seint is an adverb.

That distinction is very common in Icelandic.

In Icelandic, í can be used with expressions of duration, where English often uses for.

So:

• í smástund = for a little while

This is a standard idiomatic expression.

Other similar patterns:

• í klukkutíma = for an hour
• í tvo daga = for two days

So although í often means in or into, here it is part of a time expression meaning for.

Smástund is a noun meaning a short while or a little moment.

It is made from:

• smá- = small, little
• stund = a while, a period of time, a moment

So:

• í smástund = for a short while / for a little bit

It is a very natural everyday expression.

It is natural and normal everyday Icelandic.

A speaker might say this in ordinary conversation without sounding unusually formal or stiff.

You may also hear slightly different versions, such as:

• Ef þú kemur seint, þá bíð ég aðeins.
• Ef þú kemur seint, bíð ég smá.

But your sentence is perfectly good, clear, and natural.

Yes. Icelandic can also put the main clause first:

• Ég bíð í smástund ef þú kemur seint.

That means the same thing: I’ll wait a little while if you come late.

When the if-clause comes first, Icelandic often shows the verb-second pattern more clearly in the main clause:

• Ef þú kemur seint, þá bíð ég í smástund.

So both orders are possible, but the word order changes in the expected Icelandic way.

The letter þ is called thorn. It is pronounced like the th in thin, not like the th in this.

So:

• þú sounds roughly like thoo

A few helpful pronunciation notes for this sentence:

• þú = thoo
• kemur = roughly KYEH-mur
• seint = roughly saynt
• þá = roughly thow with the th of thin
• bíð = roughly beeth with a long ee
• smástund = roughly smau-stund

These are only rough English-based approximations, but they can help at the beginning.

Yes, but that depends on the meaning.

In your sentence, the speaker is talking about a real possible future situation, so the present tense is normal:

• Ef þú kemur seint, þá bíð ég í smástund.

If the speaker wanted to talk about something more hypothetical or unreal, Icelandic could use different tense/mood patterns.

But for basic, real conditions, this kind of present-tense structure is exactly what learners should expect and learn first.