Hon lägger bananer i frysen när hon vill göra kall yoghurt.

Because the sentence describes a habitual action: this is what she does whenever she wants to make cold yogurt. Swedish often uses the present tense for routines and general patterns.

So Hon lägger bananer i frysen när hon vill göra kall yoghurt means something like:

• she puts bananas in the freezer whenever she wants to make cold yogurt

not necessarily that she is doing it right this second.

Because Swedish normally needs an explicit subject in each clause.

This sentence has:

• a main clause: Hon lägger bananer i frysen
• a subordinate clause: när hon vill göra kall yoghurt

Each clause needs its own subject, so hon appears twice.

Lägger is the present tense of lägga, which often means put, lay, or place.

In Swedish, different verbs can be used depending on how something is positioned:

• lägga = lay/put something down, often not upright
• ställa = stand/put something upright
• sätta = set/put, often into a position

With bananer, lägger sounds natural, because bananas are not usually thought of as standing upright.

Frysen is the definite singular form of frys.

• en frys = a freezer
• frysen = the freezer

In Swedish, the definite article is often added to the end of the noun instead of being a separate word.

Because it means in the freezer, not just in a freezer.

Swedish often uses the definite form when the object is understood from context, especially for everyday household things:

• i frysen = in the freezer
• i kylskåpet = in the refrigerator
• på bordet = on the table

If you said i en frys, it would sound more general or unspecific: in a freezer.

Because vilja is a modal-type verb, and in Swedish these verbs are normally followed by the bare infinitive.

So you get:

• vill göra = wants to make/do
• kan göra = can do
• måste göra = must do

Not vill att göra.

När means when or whenever, while om means if.

Here the sentence describes something that happens whenever that situation comes up:

• när hon vill göra kall yoghurt = when/whenever she wants to make cold yogurt

If you used om, the meaning would become more conditional:

• om hon vill göra kall yoghurt = if she wants to make cold yogurt

That is possible in some contexts, but när fits better for a repeated, regular action.

Because bananer is indefinite plural.

In Swedish, indefinite plural nouns usually do not take an article:

• bananer = bananas
• äpplen = apples

If you wanted to say the bananas, you would say:

• bananerna

Because yoghurt is usually an en-word in Swedish, so the adjective takes the common gender singular indefinite form:

• kall yoghurt

Compare:

• en kall soppa
• ett kallt ägg

If the noun were an ett-word, you would use kallt instead.

Because yoghurt is being used as a mass noun, like milk or rice in English.

So:

• göra kall yoghurt = make cold yogurt

If you meant one specific yogurt, such as a cup or portion, then an article could appear:

• en kall yoghurt

But in this sentence, it is more natural without an article.

It is grammatical and understandable, but the most natural wording depends on what you mean.

• If you really mean cold yogurt, then kall yoghurt is fine.
• If you mean the dessert frozen yogurt, many speakers would more naturally say fryst yoghurt or frozen yoghurt.

So the sentence works, but in some contexts a more idiomatic phrase might be better.

Then the main clause would use inversion:

• När hon vill göra kall yoghurt lägger hon bananer i frysen.

This happens because Swedish main clauses usually have the finite verb in the second position. When the sentence begins with the när-clause, that clause takes the first position, so lägger comes before hon in the main clause.