По воскресеньям бабушка жарит оладьи на завтрак.

Because по воскресеньям means a repeated, habitual time: on Sundays / every Sunday.

A useful pattern is:

• по понедельникам = on Mondays
• по вечерам = in the evenings
• по выходным = on weekends

By contrast, в воскресенье usually means on Sunday as one specific Sunday or one occasion.

After по in this kind of time expression, Russian uses the dative plural.

The basic noun is воскресенье = Sunday.

Its relevant forms are:

• singular: воскресенье
• plural: воскресенья
• dative plural: воскресеньям

So:

• в воскресенье = on Sunday
• по воскресеньям = on Sundays / every Sunday

Not necessarily. In Russian, the present tense of an imperfective verb is often used for habitual or repeated actions.

So бабушка жарит here means something like:

• Grandma fries/makes them
• Grandma usually fries them
• Grandma does this as a routine

The phrase по воскресеньям makes it clear that this is a habit, not just something happening at this exact moment.

Because the sentence describes a regular repeated action, and Russian normally uses the imperfective for that.

Жарить = to fry, in an ongoing/repeated/habitual sense.

A perfective verb would suggest a single completed event. Also, perfective verbs do not have a true present tense in Russian; their present-looking forms usually mean future.

So:

• бабушка жарит оладьи = Grandma fries/makes pancakes regularly
• бабушка пожарит оладьи = Grandma will fry/make pancakes

Here оладьи is the accusative plural of оладья.

Because оладьи are inanimate, the accusative plural has the same form as the nominative plural.

So:

• nominative singular: оладья
• accusative singular: оладью
• nominative plural: оладьи
• accusative plural: оладьи

Since Grandma is frying more than one, the plural makes sense.

Оладьи are a kind of small, thick Russian pancake or fritter.

They are not quite the same as блины, which are usually thinner and more crepe-like.

So although English often translates оладьи as pancakes, a more precise idea is:

• small
• thick
• fluffy
• pan-fried batter cakes

Because на + accusative is the normal Russian pattern for meals in this meaning.

You will often see:

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

In this sentence, на завтрак means that the pancakes are what she makes as breakfast or for breakfast.

Yes. Russian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

This version is neutral and natural:

• По воскресеньям бабушка жарит оладьи на завтрак.

Other possible orders:

• Бабушка по воскресеньям жарит оладьи на завтрак.
  Slightly more focus on Grandma as the topic.

• На завтрак бабушка по воскресеньям жарит оладьи.
  More emphasis on for breakfast.

• Оладьи бабушка жарит по воскресеньям на завтрак.
  More emphasis on оладьи.

Russian does not have articles.

So nouns like бабушка and оладьи appear without words like the or a/an. The exact meaning is understood from context.

Depending on context, бабушка could mean:

• Grandma
• the grandmother
• a grandmother

In this sentence, it most naturally means Grandma.

Usually more like grandma.

Бабушка is the normal everyday word and often sounds warm and natural. The more formal English word grandmother can be a possible translation, but in tone grandma is often closer.

It can also sometimes mean an old woman in other contexts, but here it clearly means Grandma.

A rough English approximation is:

• оладьи ≈ a-LAD-yee
• воскресеньям ≈ va-skre-SEN-yam

Two useful pronunciation points:

• Unstressed о often sounds more like a.
• The soft sign ь in оладьи does not make its own sound; it softens the consonant before it.