Hún kreistir aðra sítrónu í vatnið þegar það er heitt úti.

It can mean either, depending on context.

In this sentence, aðra sítrónu most naturally means another lemon. But annar can also mean second in some contexts.

So:

• aðra sítrónu = another lemon
• sometimes also = a second lemon

Context decides which English wording sounds best.

Because aðra sítrónu is the direct object of the verb kreistir.

The dictionary form is:

• önnur sítróna = another lemon in the nominative

But after the verb, Icelandic needs the accusative here:

• aðra sítrónu

Both words change:

• önnur → aðra
• sítróna → sítrónu

This happens because Icelandic marks case on both the adjective/pronoun-like word and the noun.

Because sítróna is a feminine noun, and here it is in the accusative singular.

So:

• nominative: sítróna
• accusative: sítrónu

Since it is the thing being squeezed, it appears in the accusative.

A very literal grammar breakdown is:

• Hún = subject
• kreistir = verb
• aðra sítrónu = object

And the object takes accusative.

Because í can take different cases depending on whether it shows movement or location.

• í + accusative = into
• í + dative = in

Here the lemon juice is being squeezed into the water, so Icelandic uses the accusative:

• í vatnið

Compare:

• Hún kreistir sítrónu í vatnið. = She squeezes a lemon into the water.
• Sítrónan er í vatninu. = The lemon is in the water.

Also:

• vatnið = the water
• vatninu = in the water / to the water depending on case use

So the difference is both about case and meaning.

That ending is the definite article attached to the noun.

In Icelandic, the is usually not a separate word. It is added to the end of the noun.

So:

• vatn = water
• vatnið = the water

Here vatnið is the accusative/nominative singular definite form of the neuter noun vatn.

Þegar means when here. It introduces a time clause:

• þegar það er heitt úti = when it is hot outside

So the sentence says that she squeezes another lemon into the water when that condition happens.

In other contexts, þegar can sometimes feel close to whenever, depending on the sentence.

It works a lot like English it in it is hot outside.

This það is not pointing to some specific thing. It is a dummy subject used in weather and general condition expressions.

So:

• það er heitt = it is hot
• það er kalt = it is cold
• það rignir = it is raining / literally just rains

English does the same kind of thing with it.

Because in this kind of impersonal expression, Icelandic normally uses the neuter singular form of the adjective.

So:

• heitur = masculine
• heit = feminine
• heitt = neuter

In það er heitt úti, there is no real masculine or feminine noun being described. It is just the general condition it is hot, so the default form is neuter singular:

• heitt

Because úti means outside / outdoors as a location, while út usually means out / outward as a direction.

So:

• úti = outside, outdoors
• út = out, outward

In this sentence, the meaning is location:

• það er heitt úti = it is hot outside

Compare:

• Hann er úti. = He is outside.
• Hann fer út. = He goes out.

That location-vs-direction difference is very common in Icelandic.

Kreistir is present tense.

The verb is kreista = to squeeze.

Present-tense forms include:

• ég kreisti = I squeeze
• þú kreistir = you squeeze
• hún kreistir = she squeezes

So in this sentence:

• Hún kreistir = She squeezes

Notice that kreistir can be both second person singular and third person singular, but the subject hún tells you it is third person singular here.

Yes. That is very natural in Icelandic.

You can say:

• Þegar það er heitt úti, kreistir hún aðra sítrónu í vatnið.

That still means the same thing.

The important thing is Icelandic verb-second word order in the main clause. When the time clause comes first, the finite verb of the main clause still comes early:

• Þegar það er heitt úti, kreistir hún ...

Not:

• Þegar það er heitt úti, hún kreistir ...

That last version sounds wrong in standard Icelandic.

Usually it means she, as it does here.

But Icelandic also has grammatical gender, so pronouns can sometimes refer back to nouns according to their grammatical gender. That means hún can also refer to a feminine noun in the right context.

In this sentence, though, the normal reading is simply:

• Hún = she