Мама накрыла торт салфеткой.

Because салфеткой is in the instrumental case.

In this sentence, the napkin is the thing used to cover the cake, so Russian marks it with the instrumental:

• салфетка = napkin (dictionary form, nominative)
• салфеткой = with a napkin / by means of a napkin

Russian often uses the instrumental case without a preposition where English uses with.

So:

• Мама накрыла торт салфеткой. = Mom covered the cake with a napkin.

For a feminine noun like салфетка, the instrumental singular ending here is -ой:

• салфетка → салфеткой

Торт is the direct object, so it is in the accusative case. But for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• торт = nominative
• торт = accusative

That is why you do not see a change.

Compare:

• Я вижу торт. = I see a cake.
• На столе торт. = There is a cake on the table.

Same form, different case function.

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

The subject is мама, which is feminine singular, so the verb must also be feminine singular:

• накрыл = masculine singular
• накрыла = feminine singular
• накрыло = neuter singular
• накрыли = plural

So:

• Мама накрыла... = Mom covered...

This is very different from English, where past tense verbs usually do not change for gender.

The dictionary form is накрыть.

Накрыла is:

• past tense
• feminine singular
• from the verb накрыть

This verb is usually perfective, so it presents the action as completed:

• накрыла = covered / has covered in the sense of a finished action

A related imperfective verb is накрывать:

• Мама накрывала торт салфеткой. = Mom was covering / used to cover the cake with a napkin.

No. Накрыть can have several meanings depending on context.

Common meanings include:

• to cover
• to put something over something
• sometimes to set the table in expressions like накрыть на стол

So in this sentence, because we have торт and салфеткой, the meaning is clearly:

• to cover the cake with a napkin

But in another sentence:

• Мама накрыла на стол. = Mom set the table.

So learners should remember that накрыть is a context-dependent verb.

Russian has no articles.

That means Russian does not have direct equivalents of a/an and the. Whether something is definite or indefinite is usually understood from context.

So:

• Мама накрыла торт салфеткой.

can mean:

• Mom covered the cake with a napkin
• Mom covered a cake with a napkin

In real use, context tells you which one is meant.

Yes. Russian word order is much more flexible than English word order because case endings show what each word is doing.

The neutral order here is:

• Мама накрыла торт салфеткой.

But other orders are possible, for example:

• Торт мама накрыла салфеткой.
• Салфеткой мама накрыла торт.

These usually change the focus or emphasis, not the basic meaning.

Very roughly:

• Мама накрыла торт салфеткой. = neutral
• Торт мама накрыла салфеткой. = emphasis on the cake
• Салфеткой мама накрыла торт. = emphasis on the napkin

English cannot do this as freely.

Because Russian often expresses means or instrument by using the instrumental case alone, without a preposition.

So English says:

• with a napkin

But Russian can simply say:

• салфеткой

This is a very common pattern in Russian:

• писать ручкой = to write with a pen
• резать ножом = to cut with a knife
• накрыть салфеткой = to cover with a napkin

So the meaning of with is built into the case form.

Мама is in the nominative case because it is the subject of the sentence.

It answers the question who did the action?

• Мама did the covering.

Even though мама ends in -а, it is not neuter; it is a feminine noun meaning mom/mother.

So the sentence structure is:

• Мама = subject, nominative
• накрыла = verb
• торт = direct object, accusative
• салфеткой = instrumental, means/instrument

The stress is:

• Ма́ма накры́ла торт салфе́ткой.

A rough pronunciation guide:

• МА-ma na-KRY-la tort sal-FYET-koy

A few useful notes:

• ё is not written here, so it is салфе́ткой, not салфёткой
• the stress in накры́ла is on -кры́-
• the final -ой in салфеткой sounds like -koy

Listening and repeating full chunks like накрыла торт салфеткой is often more helpful than reading each word separately.

Yes. Мама most naturally means Mom or Mama, but depending on style and context, English might use Mother.

However, for everyday speech, Mom is the best match.

Compare:

• мама = mom
• мать = mother (more formal, neutral, or sometimes harsher depending on context)

So in most learner translations:

• Мама накрыла торт салфеткой. = Mom covered the cake with a napkin.