На завтрак мама испекла кекс, а вечером бабушка купила чизкейк.

На + accusative is a very common Russian pattern for meals, and it usually means for that meal.

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner/supper

So На завтрак мама испекла кекс means something like Mom baked a cake for breakfast.

This is different from saying during breakfast. If you wanted that idea, Russian would use a different expression, such as во время завтрака.

Because after на in this meaning, Russian uses the accusative case.

The noun завтрак is masculine and inanimate. In the singular, masculine inanimate nouns usually have the same form in the nominative and accusative:

• nominative: завтрак
• accusative: завтрак

So the form does not change, even though the case has changed.

Вечером is an adverbial use of the instrumental case, and it means in the evening.

Russian often uses these time words without a preposition:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So вечером бабушка купила чизкейк means in the evening Grandma bought a cheesecake.

You can sometimes use в with time expressions, but вечером is the normal natural choice here.

Both can mean and in some contexts, but а often adds a sense of contrast, comparison, or topic shift.

Here the sentence has two parallel parts:

• мама испекла кекс
• бабушка купила чизкейк

and also two different time settings:

• на завтрак
• вечером

So а works well because it loosely means something like:

• while
• whereas
• and then / and on the other hand

In this sentence, а sounds very natural because it sets the second situation against the first one.

In the past tense, Russian verbs agree with the subject in gender and number.

• мама is feminine singular, so: испекла
• бабушка is feminine singular, so: купила

Compare:

• мама испекла = Mom baked
• папа испёк = Dad baked
• они испекли = They baked

So the endings tell you something about who did the action.

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

But both nouns are masculine inanimate singular, so their accusative form is the same as their nominative form:

• кекс → кекс
• чизкейк → чизкейк

If the noun were feminine, you would often see a visible change:

• мама купила книгу = Mom bought a book

They are two different foods.

• кекс usually means a sweet baked cake, often more like a loaf cake or muffin-type cake than a big frosted celebration cake
• чизкейк is cheesecake, a borrowed word from English

So the sentence contrasts two different desserts or baked items.

A learner should also know that Russian food words do not always match English food words perfectly one-to-one, so кекс is not always exactly the same as every English use of cake.

Russian has no articles.

So:

• мама испекла кекс can mean Mom baked a cake or Mom baked the cake
• бабушка купила чизкейк can mean Grandma bought a cheesecake or the cheesecake

The exact meaning depends on context. In an isolated sentence like this, English usually uses a.

Russian word order is more flexible than English word order. Starting with На завтрак puts the time/occasion first and gives it emphasis as the setting.

So:

• На завтрак мама испекла кекс = For breakfast, Mom baked a cake.
• Мама испекла кекс на завтрак = Mom baked a cake for breakfast.

Both are possible. The version in your sentence sounds natural because it first sets the scene: for breakfast.

Both are perfective verbs:

• испечь / испекла = to bake, in the sense of completing the baking
• купить / купила = to buy, in the sense of completing the purchase

Perfective aspect presents each action as a completed whole.

So the sentence is not focusing on process; it is telling you about completed events:

• Mom baked the cake
• Grandma bought the cheesecake

If you used imperfective verbs, the meaning would shift more toward process, repetition, or background description.

Russian often leaves out possessive words when they are understood from context.

So мама often means Mom / my mom, and бабушка often means Grandma / my grandma, especially in simple everyday statements.

English usually needs my, but Russian often does not.

Not always. It can mean:

• grandmother
• sometimes an elderly woman, depending on context

In this sentence, because it appears next to мама, the most natural reading is Grandma.

A fairly literal breakdown is:

• На завтрак = for breakfast
• мама = Mom
• испекла = baked
• кекс = a cake / loaf cake
• а = while / and
• вечером = in the evening
• бабушка = Grandma
• купила = bought
• чизкейк = cheesecake

So a close natural English version would be:

For breakfast Mom baked a cake, and in the evening Grandma bought a cheesecake.

Or, if you want to reflect а a bit more:

Mom baked a cake for breakfast, while Grandma bought a cheesecake in the evening.