Hon byter skor när hennes fötter är kalla.

Hon means she and is the subject pronoun.

Hennes means her in the possessive sense.

So in this sentence:

• Hon byter skor = She changes shoes
• hennes fötter = her feet

A useful comparison:

• hon = she
• henne = her (object)
• hennes = her / hers

Byter is the present tense of byta, which often means change, switch, or replace.

So hon byter skor means that she changes shoes or switches shoes.

Depending on context, byta can also mean things like:

• byta kläder = change clothes
• byta jobb = change jobs
• byta plats = switch places

So this is a very common and useful verb.

Skor is the indefinite plural form: shoes.

Skorna is the definite plural form: the shoes.

Here, the sentence just says she changes shoes in general, not a specific pair already identified, so skor is natural.

Compare:

• Hon byter skor = She changes shoes
• Hon byter skorna = She changes the shoes

Also, in Swedish, indefinite plural nouns usually have no article, unlike English sometimes needing some or another determiner.

När means when, but in sentences about habits or repeated situations, it can often feel like whenever in English.

So this sentence can be understood as:

• She changes shoes when her feet are cold
• or more naturally in some contexts, She changes shoes whenever her feet are cold

Swedish uses när for this kind of time relationship.

This is a very important Swedish grammar point.

Sin/sitt/sina is a reflexive possessive, and it is used when the possessor is the subject of the same clause.

For example:

• Hon tvättar sina fötter = She washes her own feet

Here, hon is the subject of the clause, so sina works.

But in när hennes fötter är kalla, the subject of that clause is actually hennes fötter. Because of that, sina does not fit there.

So Swedish uses hennes instead:

• när hennes fötter är kalla = when her feet are cold

A simple rule: Use sin/sitt/sina only when it clearly refers back to the subject of the same clause.

The singular is fot = foot.

The plural is irregular:

• en fot = a foot
• foten = the foot
• fötter = feet
• fötterna = the feet

So fötter is just the normal plural form, even though it does not look regular.

Because fötter is plural, the adjective must use the plural form.

Swedish adjective agreement here is:

• kall for singular en-words
• kallt for singular ett-words
• kalla for plural

So:

• en fot är kall = a foot is cold
• ett golv är kallt = a floor is cold
• fötter är kalla = feet are cold

That is why the sentence has kalla.

Yes. Both are present tense:

• byter = changes / is changing
• är = is / are

In Swedish, just like in English, the present tense is often used for habitual actions or general situations.

So this sentence is not necessarily about one specific moment only. It can describe a habit or a usual pattern:

• She changes shoes when her feet are cold

Inside the när-clause, the word order is the normal statement order:

• hennes fötter = subject
• är = verb
• kalla = adjective

So:

• när hennes fötter är kalla = when her feet are cold

If you put the när-clause first, Swedish changes the main clause word order:

• När hennes fötter är kalla, byter hon skor.

Notice byter hon, not hon byter. That happens because Swedish main clauses usually follow the verb-second rule.

Swedish often uses the definite form for body parts when ownership is obvious, especially with actions:

• Hon tvättar händerna = She washes her hands

But in this sentence, the speaker is describing a condition and explicitly identifying whose feet they are, so hennes fötter is very natural.

You could sometimes say fötterna if the context already makes it obvious whose feet are being talked about, but hennes fötter is clear and perfectly normal here.

The sentence is grammatical and understandable.

A Swede might also say something more idiomatic depending on context, for example:

• Hon byter skor när hon fryser om fötterna.

That literally means something like She changes shoes when she feels cold in her feet.

So the original sentence is fine, but Swedish also has other natural ways to express the same idea. The exact choice depends on whether you want to focus on her feet being cold or on her feeling cold in her feet.