В магазине я купила миндаль и фундук, потому что хочу испечь торт к выходным.

Because купила is the past tense form used by a feminine singular subject.

• я купила = a woman is speaking
• я купил = a man is speaking

In Russian, past tense verbs agree with the subject in gender and number. That is why this verb shows that the speaker is female.

Yes, Russian often leaves out personal pronouns when the subject is clear from the verb form or from context.

So both of these are possible:

• В магазине я купила миндаль и фундук...
• В магазине купила миндаль и фундук...

Including я can make the sentence a little clearer, more explicit, or slightly more contrastive, especially if the speaker is emphasizing I.

В магазине uses the prepositional case because it shows location: in / at the store.

• dictionary form: магазин
• after в for location: в магазине

Compare:

• в магазине = in the store
• в магазин = into the store

So here it is prepositional because the speaker is talking about where the buying happened, not movement toward the store.

This is a question of aspect.

• купила is perfective
• покупала is imperfective

Купила presents the action as completed: the speaker went to the store and finished buying the nuts.

Покупала would sound more like:

• the process of buying
• repeated buying
• background information
• an action without focus on completion

In this sentence, the completed result matters, so купила is the natural choice.

In Russian, names of foods and ingredients are very often used in the singular to mean some amount of that item, not necessarily one piece.

So:

• купила миндаль = bought some almonds / almond product
• купила фундук = bought some hazelnuts / hazelnut product

This is very natural in shopping and cooking contexts. Russian is treating them more like substance or product names here than as individually counted nuts.

They are in the accusative case because they are direct objects of купила.

However, both are inanimate masculine nouns, and for inanimate masculine singular nouns, the accusative form is the same as the nominative form.

So:

• nominative: миндаль, фундук
• accusative: миндаль, фундук

That is why there is no visible ending change.

After хотеть (to want), Russian usually uses the infinitive, just like English uses want to + verb.

So:

• хочу испечь = I want to bake
• хочу читать = I want to read
• хочу пойти = I want to go

This is a very standard pattern: хотеть + infinitive

Again, this is about aspect.

• печь = imperfective, the activity of baking
• испечь = perfective, to bake something successfully to completion

Here the speaker wants a finished result: a cake ready by the weekend. That is why испечь fits better.

So:

• хочу печь would focus more on the activity in general
• хочу испечь торт focuses on completing one cake

Торт is the direct object of испечь, so it is in the accusative case.

Since торт is an inanimate masculine singular noun, its accusative form is the same as its nominative form:

• nominative: торт
• accusative: торт

So испечь торт is exactly what you expect.

Торта would be genitive, which is used in other situations, for example:

• кусок торта = a piece of cake

It means something like by the weekend or for the weekend.

The preposition к can be used with time expressions to show a target point in time, especially when something should be ready by then.

So испечь торт к выходным means the speaker wants the cake to be ready when the weekend arrives.

Because выходные is a plural-only word in this meaning.

In Russian, выходные literally means days off, and it is commonly used to mean the weekend. Since it is plural in form, after к it takes the dative plural:

• nominative: выходные
• dative after к: к выходным

So the ending -ым comes from the dative plural.

They are similar, but not identical.

• к выходным = by the weekend, ready for the weekend
• на выходные = for the weekend period / during the weekend / to use on the weekend

In this sentence, к выходным is better because the idea is that the cake should be finished in time for the weekend.

Compare:

• испечь торт к выходным = bake a cake so it is ready by the weekend
• уехать на выходные = leave for the weekend

Because потому что introduces a subordinate clause giving the reason.

Main clause: В магазине я купила миндаль и фундук

Reason clause: потому что хочу испечь торт к выходным

In Russian, a subordinate clause introduced by потому что is normally separated by a comma.

No, Russian word order is fairly flexible. The sentence begins with В магазине to set the scene first: as for where this happened, it was in the store.

Other orders are possible, for example:

• Я купила миндаль и фундук в магазине...
  More neutral, straightforward.

• Миндаль и фундук я купила в магазине...
  Puts extra emphasis on what was bought.

• В магазине я купила миндаль и фундук...
  Puts the location first.

The basic meaning stays the same, but the emphasis and information flow change.