На полке лежал сладкий пончик, но дочка выбрала пирожное.

Because на полке expresses location: something is on the shelf.

With на, Russian often uses:

• Prepositional case for location: на полке = on the shelf
• Accusative case for motion toward a place: на полку = onto the shelf

So:

• На полке лежал пончик = The doughnut was lying on the shelf.
• Положили пончик на полку = They put the doughnut onto the shelf.

Russian often uses specific position verbs where English might simply say was.

• лежать = to lie
• стоять = to stand
• сидеть = to sit

So На полке лежал сладкий пончик literally describes the doughnut as lying on the shelf. This sounds natural in Russian.

English often says:

• There was a doughnut on the shelf

Russian commonly prefers:

• На полке лежал пончик

Because лежал agrees with пончик, which is masculine singular.

In the past tense, Russian verbs agree in gender and number:

• masculine: лежал
• feminine: лежала
• neuter: лежало
• plural: лежали

Since пончик is masculine, the verb is лежал.

They are both in the nominative singular masculine form because сладкий пончик is the subject of the first clause.

• пончик = masculine singular noun
• сладкий = adjective agreeing with it

Adjectives in Russian must match the noun in:

• gender
• number
• case

So:

• сладкий пончик = nominative masculine singular

The verb выбрала is past tense and agrees with дочка, which is feminine singular.

Past tense forms:

• выбрал = masculine
• выбрала = feminine
• выбрало = neuter
• выбрали = plural

Since дочка is feminine, Russian uses выбрала.

Because выбрала is perfective, and it presents the choice as a completed action.

• выбрать → perfective = to choose, to make a choice
• выбирать → imperfective = to be choosing, to choose repeatedly, to focus on the process

In this sentence, the daughter made her choice, so выбрала is the natural form.

Compare:

• дочка выбрала пирожное = the daughter chose the pastry
• дочка выбирала пирожное = the daughter was choosing / spent time choosing a pastry

Because пирожное is a neuter inanimate noun, and for inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: пирожное
• accusative: пирожное

Here it is the direct object of выбрала, so it is in the accusative case, but the form does not change.

Russian word order is flexible, and the beginning of the sentence often sets the scene or highlights known/background information.

Starting with На полке puts the location first:

• On the shelf, there was a sweet doughnut...

This is very natural in Russian. It sounds like the sentence is first establishing where the doughnut was, then telling you what happened next.

A different order is possible:

• Сладкий пончик лежал на полке...

But На полке лежал... often sounds more natural when introducing something in a place.

Because но means but, and it connects two clauses:

• На полке лежал сладкий пончик
• дочка выбрала пирожное

In Russian, но is normally preceded by a comma when it joins two parts like this.

So the comma is standard punctuation here.

Both mean daughter, but дочка is more everyday, affectionate, or informal.

• дочь = the basic dictionary form, more neutral or formal
• дочка = common in speech, warmer, more personal

So дочка выбрала пирожное sounds natural and conversational.

Но means but and introduces a contrast.

The contrast is:

• there was a sweet doughnut on the shelf,
• but the daughter chose a pastry instead.

It signals that the second part goes against what you might expect. Even though the doughnut was there, she picked something else.

Grammatically, it is not necessary, but it adds description.

• На полке лежал пончик = plain statement
• На полке лежал сладкий пончик = more descriptive

For a learner, it is also a useful example of adjective agreement:

• сладкий matches пончик in masculine singular nominative.