После завтрака дети оставили крошки на диване, и мама начала ворчать.

Because после requires the genitive case.

• завтрак = breakfast (dictionary form, nominative)
• After после, it becomes завтрака (genitive singular)

So:

• после завтрака = after breakfast
• Compare:
  • во время завтрака = during breakfast
  • до завтрака = before breakfast

This is a very common pattern in Russian: many prepositions force a particular case, and после always takes the genitive.

Дети is an irregular plural of ребёнок (child) or related to дитя in older forms. It means children.

So:

• ребёнок = child
• дети = children

This is just something you have to memorize, much like child → children in English.

In this sentence, дети is in the nominative plural because it is the subject of the verb:

• дети оставили = the children left

Оставили is the past tense plural form of оставить.

Russian past tense agrees with the subject in gender and number:

• masculine singular: оставил
• feminine singular: оставила
• neuter singular: оставило
• plural: оставили

Since дети is plural, the verb must also be plural:

• дети оставили = the children left

This is a question of aspect.

• оставить = perfective
• оставлять = imperfective

In this sentence, оставили presents the action as a completed event: the children left crumbs on the sofa, and that happened as a finished action.

• дети оставили крошки = the children left crumbs

If you used оставляли, it would sound more like:

• they used to leave crumbs
• they were leaving crumbs
• the action is viewed as ongoing, repeated, or not focused on completion

Here the sentence is telling a simple sequence of events, so perfective makes sense.

Because крошки means crumbs, plural.

The singular is:

• крошка = a crumb

The plural is:

• крошки = crumbs

In the sentence, крошки is the direct object of оставили. For an inanimate noun like this, the plural accusative looks the same as the plural nominative:

• nominative plural: крошки
• accusative plural: крошки

So the form stays the same.

Because this sentence describes location, not direction.

• на диване = on the sofa / on the couch → location
• на диван = onto the sofa → movement toward it

Russian uses different cases here:

• на + prepositional for location
• на + accusative for destination/movement

So:

• крошки на диване = the crumbs are on the sofa
• положить крошки на диван = to put crumbs onto the sofa

In your sentence, the crumbs ended up being located there, so на диване is correct.

Because the preposition на can take different cases depending on meaning.

When на means on/at in the sense of location, it usually takes the prepositional case:

• на столе = on the table
• на полу = on the floor
• на диване = on the sofa

That is why:

• диван → диване

This is one of the most useful case patterns to learn early.

Начала ворчать means began to grumble or started grumbling. It emphasizes the beginning of the action.

• мама начала ворчать = mom started grumbling
• мама ворчала = mom was grumbling / used to grumble / grumbled

So the sentence gives a sequence:

1. the children left crumbs on the sofa
2. then mom started grumbling

Using начала ворчать makes that sequence very clear.

Because the subject is мама, which is feminine singular.

In the past tense, Russian verbs agree with the subject:

• он начал = he started
• она начала = she started
• они начали = they started

Since мама is feminine singular:

• мама начала

Because after начать (to begin/start), Russian normally uses an infinitive to express what action began.

So:

• начать ворчать = to start grumbling
• начать читать = to start reading
• начать говорить = to start speaking

This is very similar to English start + -ing or begin to + verb, except Russian uses the infinitive.

Because и here joins two separate clauses, each with its own subject and verb:

1. дети оставили крошки на диване
2. мама начала ворчать

Since these are two full clauses, Russian normally uses a comma before и.

Compare:

• Дети ели и смеялись.
  No comma, because this is one subject with two verbs.

• Дети ели, и мама улыбалась.
  Comma, because these are two clauses with different subjects.

Yes. Russian word order is flexible, because cases show grammatical relationships. The neutral order here is very natural:

• После завтрака дети оставили крошки на диване, и мама начала ворчать.

But other orders are possible for emphasis, for example:

• Дети после завтрака оставили крошки на диване, и мама начала ворчать.
• На диване дети после завтрака оставили крошки, и мама начала ворчать.

These versions may sound more marked or emphasize different parts of the sentence.

So yes, the order can change, but the original version is the most straightforward and natural in ordinary narration.

Мама usually means mom, mum, or mommy depending on context and tone. It is more personal and everyday than мать, which is more formal or neutral and can sound less warm in many contexts.

So in this sentence:

• мама начала ворчать feels natural and domestic: Mom started grumbling

If you said мать начала ворчать, it would usually sound different in tone and often less natural for a simple family scene like this.

Yes, a few are worth noticing:

• после
• зАВтрака? Careful: завтрАка
• дЕти
• остАВили
• крОшки
• на дивАне
• мАма
• началА
• ворчАть

A particularly useful one to remember is:

• началА = she began

Many learners want to stress it incorrectly, so it is good to memorize this form early.

Because Russian has no articles.

English says:

• the children
• crumbs
• the sofa
• mom

Russian simply uses the noun itself, and the exact meaning is understood from context:

• дети
• крошки
• диван
• мама

So when reading Russian, you often have to supply a, an, or the naturally in English based on context.