На чизкейке была тонкая глазурь, а на пирожном — шоколадная посыпка.

Here на means on in the sense of on the surface of something.

• на чизкейке = on the cheesecake
• на пирожном = on the pastry/cake

Russian commonly uses на + prepositional case for location on a surface. Since the icing and topping are located on the desserts, на is the natural choice.

Because after на in a location meaning, Russian uses the prepositional case.

So:

• чизкейк → на чизкейке
• пирожное → на пирожном

These are just the case forms required by на when it means on/in a place.

Была agrees with глазурь, which is a feminine singular noun.

• глазурь = feminine
• therefore: была

Russian past-tense verbs change for gender in the singular:

• masculine: был
• feminine: была
• neuter: было
• plural: были

So На чизкейке была тонкая глазурь literally has a feminine subject, and the verb matches it.

The dash shows that the verb была is being left out because it is understood.

Full version:

На чизкейке была тонкая глазурь, а на пирожном была шоколадная посыпка.

Russian often omits repeated words when they are obvious. The dash marks that omission. This makes the sentence more natural and less repetitive.

So the dash here basically means:

а на пирожном — [была] шоколадная посыпка

Here а is contrastive. It often means something like:

• and
• while
• whereas
• as for

It connects two parallel ideas and highlights a difference between them:

• on the cheesecake: thin icing
• on the pastry: chocolate topping

So it is not a strong but, but it does show contrast.

They are adjectives agreeing with feminine singular nouns:

• тонкая agrees with глазурь
• шоколадная agrees with посыпка

In Russian, adjectives must match the noun in:

• gender
• number
• case

Both глазурь and посыпка are feminine singular nominative, so the adjectives are too.

That is a very good question. Пирожное is one of those Russian nouns that historically come from adjectives and still decline like adjectives.

So its forms look like adjective forms:

• nominative: пирожное
• prepositional: на пирожном

That is why you get -ом, not a more typical noun ending. Learners often notice this because it looks unusual if they expect a standard neuter noun pattern.

Yes. In this sentence, the things being talked about are:

• тонкая глазурь
• шоколадная посыпка

They are the grammatical subjects, which is why they are in the nominative case.

The sentence structure is basically:

• На чизкейке = location
• была тонкая глазурь = there was thin icing

and then:

• на пирожном = location
• [была] шоколадная посыпка = there was chocolate topping

Russian word order is flexible, and this order is very natural when you want to present where something was first.

Starting with На чизкейке sets the scene:

• On the cheesecake there was thin icing,
• and on the pastry chocolate topping.

This kind of word order is common when describing what is present in different places. It sounds informative and natural, not unusual.

In the past tense, normally no: you need была.

But in the present tense, Russian usually leaves out есть:

• На чизкейке тонкая глазурь, а на пирожном — шоколадная посыпка.

That would mean:

There is thin icing on the cheesecake, and chocolate topping on the pastry.

So:

• present: verb often omitted
• past: был / была / было / были is usually stated

They refer to different kinds of dessert toppings.

• глазурь = icing, glaze, a smooth coating
• посыпка = sprinkles, a sprinkled topping, something scattered over the surface

So the sentence contrasts two different textures/types of topping:

• a thin glaze/icing
• a chocolate sprinkled topping

The exact English translation can vary a bit by context, but the Russian distinction is clear.

Yes. Чизкейк is a normal borrowed word in modern Russian, especially in café, bakery, and restaurant contexts.

Even though it is borrowed from English, it behaves like a regular Russian masculine noun:

• чизкейк
• чизкейка
• чизкейку
• на чизкейке, etc.

So in this sentence it is fully integrated into Russian grammar.

In Russian, nouns ending in -ь can be either masculine or feminine, so you often just have to learn the gender with the word.

Глазурь is feminine, which is why you get:

• тонкая глазурь
• глазурь была

This is a common issue for learners, because the soft sign alone does not always tell you the gender.