Ef hún vaknar á nóttunni, þarf ég stundum að skipta á henni.

Ef means if, and in ordinary real-life conditions Icelandic usually uses the present tense indicative, just like here:

• ef hún vaknar = if she wakes up

So vaknar is the normal 3rd person singular present form of vakna (to wake up).

You may sometimes see þá (then) in the main clause after an ef-clause, but it is often omitted:

• Ef hún vaknar, þarf ég...
• Ef hún vaknar, þá þarf ég...

Both are possible, but leaving out þá is very common.

Because hún is she, and the verb is in 3rd person singular present.

The verb is að vakna = to wake up. Its present-tense forms are:

• ég vakna = I wake up
• þú vaknar = you wake up
• hún/hann/það vaknar = she/he/it wakes up

So hún vaknar simply means she wakes up.

Á nóttunni means at night / during the night.

Breakdown:

• á = on, at, in
• nóttunni = the night in the dative singular

This expression is idiomatic. Icelandic often uses the definite form here:

• á nóttunni = at night / in the night

The base noun is:

• nótt = night

After the preposition á in this time expression, the noun appears in the dative:

• nótt → nóttunni

So this is not a word-for-word English pattern; it is just the normal Icelandic way to say at night in this context.

This is because of verb-second word order in Icelandic.

The sentence starts with a subordinate clause:

• Ef hún vaknar á nóttunni = If she wakes up at night

After that clause, the finite verb of the main clause comes first:

• þarf ég

So Icelandic says:

• Ef hún vaknar á nóttunni, þarf ég...

not

• Ef hún vaknar á nóttunni, ég þarf...

This is a very important Icelandic pattern. When something other than the subject comes first, the finite verb usually comes in the second position.

Because þurfa (to need / have to) is followed by an infinitive, and Icelandic normally uses að before that infinitive.

So:

• þarf ég að skipta = I need to change
• literally: need I to change

This is very common with many verbs:

• ég ætla að fara = I intend to go
• ég þarf að sofa = I need to sleep

This is a very important vocabulary point.

Að skipta by itself often means things like:

• to divide
• to split
• to exchange
• to change

But að skipta á einhverjum is a fixed expression meaning:

• to change someone
• especially to change a baby’s diaper / nappy

So in this sentence:

• að skipta á henni = to change her (meaning her diaper/nappy)

You should learn skipta á einhverjum as a whole expression.

Henni is the dative singular form of hún (she/her).

The expression að skipta á einhverjum requires the person being changed to be in the dative, because of the preposition á in this idiom.

Pronoun forms for hún are:

• hún = nominative
• hana = accusative
• henni = dative
• hennar = genitive

So:

• að skipta á henni = to change her

Even though English just says her, Icelandic must choose the correct grammatical case, and here that case is dative.

Stundum means sometimes, and its position here is very natural:

• þarf ég stundum að skipta á henni

It often goes after the finite verb and subject, but adverb placement in Icelandic can be somewhat flexible depending on emphasis and style.

This sentence sounds normal and idiomatic as written. For a learner, the safest pattern is:

• subordinate clause
• finite verb
• subject
• adverb
• infinitive phrase

So:

• Ef hún vaknar á nóttunni, þarf ég stundum að skipta á henni.

Grammatically, hún just means she, so it could refer to any female person.

However, the phrase að skipta á henni strongly suggests a baby or small child, because that expression usually means to change her diaper/nappy.

So the grammar itself does not force baby, but the vocabulary makes that interpretation very likely.

Yes—this sentence contains several very useful Icelandic patterns:

1. Ef + present tense for real conditions

   • Ef hún vaknar... = If she wakes up...

2. Verb-second word order in the main clause

   • ..., þarf ég...

3. Þurfa + að + infinitive

   • þarf að skipta

4. A fixed idiom with a preposition

   • skipta á einhverjum = change someone’s diaper/nappy

5. Dative after the idiom

   • henni

So this is an excellent sentence for noticing how Icelandic often works through set phrases, case forms, and word order, rather than by translating word for word from English.