Ef barnið sér skel aftur, mun það líklega gefa ömmu hana.

This is because Icelandic follows the V2 rule in main clauses: the finite verb must come in the second position.

Here, the whole if-clause counts as the first element:

• Ef barnið sér skel aftur = first element
• so the finite verb comes next: mun
• then the subject: það

So:

• Ef barnið sér skel aftur, mun það líklega gefa ömmu hana.

If you started the clause on its own, without the if-clause, you would normally get:

• Það mun líklega gefa ömmu hana.

Mun is the singular form of the verb munu, which is often used to express the future.

So:

• mun gefa = will give

Literally, the structure is something like:

• mun = will
• gefa = give

Icelandic can sometimes use the present tense for future meaning too, but munu + infinitive is a very common clear way to express a future event.

Yes grammatically, but not necessarily in the English sense.

The noun barn (child) is a neuter noun in Icelandic. Because of that, when you refer back to it with a pronoun, you use the neuter pronoun:

• barnið = the child
• það = it

This is about grammatical gender, not about whether the child is literally being treated as an object. Icelandic often uses pronouns that match the noun’s grammatical gender.

Because they mean different things in the sentence:

• barnið = the child
• skel = a shell

Icelandic usually expresses definiteness by attaching the article to the noun:

• barn = child
• barnið = the child

But an indefinite noun often appears with no article at all:

• skel = a shell / shell

So skel here is just an indefinite direct object.

Because hana refers back to skel.

• skel = shell
• hana = her / it, feminine accusative singular

The noun skel is feminine, so when it is replaced by a pronoun, the pronoun must also be feminine:

• skel → hana

In English we would say it, but Icelandic pronouns reflect grammatical gender, so a feminine noun takes hana in the accusative.

Ömmu is dative singular of amma (grandmother / grandma).

The verb gefa (to give) usually takes:

• the person receiving something in the dative
• the thing being given in the accusative

So in this sentence:

• ömmu = to grandma / grandma as recipient
• hana = it, the thing being given

That is why you get:

• gefa ömmu hana = give grandma it

With gefa, the normal order is often:

• indirect object first
• direct object second

So:

• ömmu = indirect object, dative
• hana = direct object, accusative

That makes gefa ömmu hana the most natural order here.

Because Icelandic has case marking, other orders can sometimes occur for emphasis or style, but ömmu hana is the straightforward neutral order.

You could repeat the noun, but using a pronoun is more natural once the thing has already been mentioned.

So the sentence first introduces the object:

• skel = a shell

Then later refers back to it with:

• hana = it

This works just like English often prefers it instead of repeating the shell.

Sér is the 3rd person singular present tense of the verb sjá (to see).

So:

• að sjá = to see
• ég sé = I see
• þú sérð = you see
• hann / hún / það sér = he / she / it sees

Since the subject is barnið (the child), which is third person singular, the correct form is sér.

Aftur means again, and adverbs like this are often placed after the verb’s object or later in the clause.

So:

• Ef barnið sér skel aftur = If the child sees a shell again

That placement is natural Icelandic. It tells you that the seeing happens again.

In practice, adverb placement can be somewhat flexible, but this order is very normal.

Líklega means probably. In this sentence it comes after the subject:

• mun það líklega gefa ...

That is a very natural position for a sentence adverb.

You may also see adverbs like this in slightly different positions depending on emphasis and style, but here the structure is very standard:

• finite verb: mun
• subject: það
• adverb: líklega
• infinitive: gefa

This is similar to English.

In English we say:

• If the child sees a shell again, it will probably give it to grandma.

We do not normally say:

• If the child will see a shell again...

Icelandic works similarly here. The if-clause uses present tense:

• Ef barnið sér ... = If the child sees ...

Then the main clause can show future meaning:

• mun það líklega gefa ... = it will probably give ...

So the time reference is future overall, even though the verb inside the if-clause is present tense.

Yes, in some contexts Icelandic can use the present tense to talk about the future, especially when the context already makes the future meaning clear.

So a version without mun could still be understandable. But mun makes the future meaning more explicit and often sounds clearer in a sentence like this.

So:

• Ef barnið sér skel aftur, gefur það líklega ömmu hana.

could be understandable in context, but

• Ef barnið sér skel aftur, mun það líklega gefa ömmu hana.

more clearly means will probably give.