Ef þú ferð núna, sleppur þú líklega við lengstu röðina í búðinni.

That is normal in Icelandic. The sentence uses present-tense forms, ferð and sleppur, but the meaning can still be future.

With ef clauses, Icelandic often uses the present tense for a real or likely future condition:

Ef þú ferð núna, sleppur þú líklega...

A natural English translation is: If you go now, you’ll probably avoid...

So the grammar is present tense, but the meaning is future-oriented.

Because ferð is the 2nd person singular present form of fara.

Present tense of fara:

• ég fer = I go
• þú ferð = you go
• hann/hún/það fer = he/she/it goes

So since the subject is þú, the correct form is ferð.

In this sentence, fara most naturally means go or leave: Ef þú ferð núna = If you go/leave now

The sentence has two clauses, and each clause has its own subject:

• Ef þú ferð núna
• sleppur þú líklega við lengstu röðina í búðinni

Icelandic normally states the subject in both clauses, just like English does:

If you go now, you will probably avoid...

So the second þú is not extra; it belongs to the main clause.

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

When the sentence starts with something other than the main-clause subject—in this case the whole ef clause—then the finite verb comes before the subject in the main clause:

Ef þú ferð núna, sleppur þú líklega...

If the main clause stood by itself, the order would normally be:

Þú sleppur líklega við lengstu röðina í búðinni.

So the fronted if clause triggers the inversion.

Sleppa við is a very common combination meaning:

• avoid
• escape
• not have to deal with

So here:

sleppur þú líklega við lengstu röðina means you will probably avoid the longest line

This is worth learning as a unit, because sleppa by itself can have other meanings too, such as let go, release, or get away.

Because the verb phrase is sleppa við, not just sleppa.

In this construction, við is part of the meaning:

• sleppa við eitthvað = avoid something

So við lengstu röðina means something like having to deal with the longest line.

This is one of those verb + preposition combinations that you mostly have to learn as a set.

Because this phrase is:

• feminine
• singular
• accusative
• definite

Here is what is happening:

• röð = line, queue
• röðina = the line (accusative singular definite)
• langur = long
• lengstur = longest

Since the adjective comes before a definite noun, it takes the weak form:

• lengstu röðina = the longest line

So the ending -u on lengstu matches the grammatical form of röðina.

Because við takes the accusative here, and röðina is the accusative singular definite form of röð.

So:

• röð = a line / queue
• röðina = the line

The whole phrase after við is the thing being avoided, so it appears in the accusative.

In this sentence, röð means line or queue—the line of people in the store.

So:

• lengsta röðin = the longest line / queue
• í búðinni shows that this is the line in the store

Depending on context, röð can also mean row or order, but here queue is clearly the right sense.

Because í uses different cases depending on meaning:

• accusative for movement into
• dative for location in

Here the meaning is location: the line in the store

So Icelandic uses the dative: í búðinni

Compare:

• Ég er í búðinni = I am in the store
• Ég fer í búðina = I go into the store

Líklega means probably.

In this sentence it comes after the subject in the main clause:

sleppur þú líklega við...

That placement is very natural in Icelandic. Because of verb-second order, the finite verb comes first in the main clause, then the subject, and then an adverb like líklega often follows.

Without the fronted ef clause, you would typically get: Þú sleppur líklega við lengstu röðina í búðinni.

Because Icelandic usually separates a fronted subordinate clause from the main clause with a comma.

So:

Ef þú ferð núna, sleppur þú líklega...

This is similar to English punctuation in: If you go now, you’ll probably avoid...

So the comma is normal and helps show where the if clause ends and the main clause begins.