Hún var enn syfjuð og geispaði þegar ég vakti hana, en eftir heita sturtu byrjaði hún að teygja sig og brosa.

Because the adjective has to agree with hún, which is feminine singular nominative.

The dictionary form is usually the masculine, syfjaður. Its forms are:

• syfjaður — masculine
• syfjuð — feminine
• syfjað — neuter

Since the subject is hún (she), Icelandic uses the feminine form: Hún var enn syfjuð.

Enn means still here.

So var enn syfjuð means she was still sleepy. It adds the idea that the sleepiness had not gone away yet.

A useful comparison:

• enn = still / yet
• þá enn or similar combinations can strengthen that idea in other contexts

In this sentence, enn modifies syfjuð.

Geispaði is the past tense of the verb að geispa (to yawn).

So:

• að geispa — to yawn
• geispar — yawns / is yawning
• geispaði — yawned

It is a very common weak-verb past-tense pattern in Icelandic.

Because þegar introduces a subordinate clause, and subordinate clauses do not follow the same verb-second pattern as main clauses.

So after þegar, the normal order is:

• þegar ég vakti hana — when I woke her

not:

• þegar vakti ég hana

That second pattern would sound wrong in standard Icelandic.

This is a very important contrast in Icelandic:

• main clause: often verb-second
• subordinate clause: usually subject before the finite verb

Because að vekja / vakta? Wait—here the verb is að vekja in modern standard Icelandic? Actually the form vakti here is from að vekja meaning to wake someone. In everyday language, vakti hana means woke her.

Compare:

• að vakna — to wake up, become awake oneself
• að vekja — to wake someone

So:

• Hún vaknaði — She woke up.
• Ég vakti hana — I woke her.

This is the same basic distinction as English wake up vs wake someone.

Hana is the accusative singular form of hún.

That is used because she is the direct object of the verb vakti:

• ég = subject
• vakti = verb
• hana = object

The pronoun forms are:

• hún — she
• hana — her

So Icelandic shows the grammatical role by case, not just by word order.

Here eftir means after in a time sense, and with that meaning it takes the accusative.

So both the adjective and the noun appear in accusative singular:

• heita — feminine accusative singular of the adjective
• sturtu — accusative singular of sturta

That is why you get eftir heita sturtu.

Also, there is no definite article because the phrase is indefinite: after a hot shower, not after the hot shower.

So:

• eftir heita sturtu — after a hot shower
• eftir heitu sturtuna or other forms would mean something different and would not fit this sentence

Because main clauses in Icelandic usually follow the verb-second rule.

When a phrase is moved to the front of the sentence, the finite verb still comes next. So:

• Eftir heita sturtu byrjaði hún...

Here the first element is eftir heita sturtu, and then the finite verb byrjaði comes second. The subject hún comes after that.

This kind of inversion is very normal in Icelandic and is something English speakers often need to get used to.

Because að byrja commonly takes an infinitive with að:

• byrja að gera eitthvað — begin to do something

So:

• byrjaði hún að teygja sig og brosa — she began to stretch and smile

Only one að is needed here because it governs both infinitives:

• að teygja sig
• og brosa

You could think of it as: began to stretch and smile. English works the same way.

Because að teygja sig is the normal reflexive expression for to stretch oneself / to stretch.

The word sig is the reflexive pronoun, used when the subject and object are the same person.

So:

• hún teygði sig — she stretched
• literally, she stretched herself

In many cases, English does not say herself, but Icelandic often does.

Because when two infinitives are connected by og, Icelandic often uses að only once if both infinitives depend on the same verb.

So this is completely normal:

• byrjaði að teygja sig og brosa

It would be possible in some contexts to repeat að for emphasis or clarity, but it is not necessary here.

Icelandic often uses the simple past where English might prefer a past continuous or another more specific past expression.

In this sentence:

• var — was
• geispaði — yawned
• vakti — woke
• byrjaði — began

The timing and flow are understood from context, not from a large tense system. Icelandic can express ongoing action, but it often does not need a separate form equivalent to English was yawning.

So a plain past-tense verb in Icelandic can sound more natural where English wants something more detailed.

En means but, so it introduces a contrast.

The sentence contrasts two states:

• before: she was still sleepy and yawning
• after the shower: she began to stretch and smile

If the sentence used og there, it would sound more like simple addition. En signals that the second part goes in a different direction.