Мама посадила ребёнка на стул.

Here посадила means sat/seated (someone) down or put (someone) onto a seat.

Grammatically, it is:

• the past tense
• feminine singular
• from the verb посадить

A very useful point: посадить is often a causative idea — not to sit, but to make someone sit / seat someone.

So:

• ребёнок сел = the child sat down
• мама посадила ребёнка = the mother sat the child down / seated the child

Because the subject is мама, which is feminine, the past tense form is посадила.

Мама is the subject of the sentence, so it is in the nominative case.

In Russian, the subject of a sentence is normally in the nominative:

• Мама = mother / mom
• Мама посадила ребёнка на стул. = Mom seated the child on the chair.

Also, the verb form посадила matches a feminine singular subject, which fits мама.

Because ребёнка is the direct object of the verb посадила.

The mother is doing the action to the child, so the child must be in the accusative case.

The noun ребёнок is masculine and animate. For many animate masculine nouns, the accusative singular looks like the genitive singular:

• nominative: ребёнок
• accusative: ребёнка

So:

• ребёнок = child as subject
• вижу ребёнка = I see the child
• мама посадила ребёнка = Mom seated the child

This is a very important Russian pattern:
animate masculine direct objects often change form in the accusative.

Because this sentence expresses motion toward a place, not location in a place.

With на, Russian usually works like this:

• на + accusative = onto / to
• на + prepositional = on / at (location)

So:

• на стул = onto the chair / to the chair
• на стуле = on the chair (already there)

In this sentence, the child is being moved into a sitting position onto the chair, so Russian uses на стул.

Compare:

• Мама посадила ребёнка на стул. = Mom seated the child on the chair.
• Ребёнок сидит на стуле. = The child is sitting on the chair.

It can mean several different things depending on context.

Common meanings of посадить include:

• to seat / sit someone down
• to plant
  
  • посадить дерево = to plant a tree
• to put someone in prison / jail
• sometimes more generally to place/set something down

In your sentence, because of ребёнка на стул, the meaning is clearly to sit the child down on the chair.

So yes, посадить is a multi-purpose verb, and the object plus context tell you which meaning is intended.

Because Russian has no articles.

English distinguishes:

• a child
• the child

Russian usually does not mark that difference with a separate word. Instead, the meaning comes from:

• context
• word order
• intonation
• the situation being discussed

So Мама посадила ребёнка на стул could be understood in English as:

• Mom seated the child on a chair
• Mom seated the child on the chair
• Mom sat the child down on a chair
• Mom sat the child down on the chair

The exact English article depends on context, not on a specific Russian word.

Russian word order is more flexible than English word order because case endings show grammatical roles.

In this sentence:

• Мама is the subject
• ребёнка is the object
• на стул shows destination

So Russian can rearrange the words more easily than English, for example:

• Мама посадила ребёнка на стул.
• Ребёнка мама посадила на стул.
• На стул мама посадила ребёнка.

These versions all mean roughly the same thing, but the focus changes:

• Мама посадила ребёнка на стул. = fairly neutral
• Ребёнка мама посадила на стул. = emphasizes the child
• На стул мама посадила ребёнка. = emphasizes onto the chair

So yes, the word order matters for emphasis, but not as rigidly as in English.

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

For singular subjects:

• masculine: usually no extra -a
  
  • посадил
• feminine: -а
  
  • посадила
• neuter: -о
  
  • посадило

Plural:

• посадили

Since мама is feminine singular, the verb becomes посадила.

Examples:

• Папа посадил ребёнка на стул. = Dad seated the child on the chair.
• Мама посадила ребёнка на стул. = Mom seated the child on the chair.

This is a very common learner question.

• сесть = to sit down
  The subject does the action themselves.
• садиться = to be sitting down / sit down in the imperfective sense
• посадить = to seat someone / make someone sit down

Compare:

• Ребёнок сел на стул. = The child sat down on the chair.
• Мама посадила ребёнка на стул. = Mom seated the child on the chair.

So сесть is intransitive here, while посадить is transitive and usually takes an object.

Посадила comes from посадить, which is perfective.

Perfective verbs usually present an action as:

• completed
• single
• result-oriented

So Мама посадила ребёнка на стул suggests a completed event:
the mother successfully seated the child.

The imperfective partner is usually сажать:

• Мама сажала ребёнка на стул.

That could suggest:

• repeated action
• process
• background description
• attempted or ongoing action, depending on context

So the perfective посадила is very natural if you mean she sat the child down as one completed action.

It is pronounced roughly ree-BYON-ka.

The letter ё always indicates a yo sound, and it is stressed here:

• ребёнок
• ребёнка

A useful thing to know: in many printed texts, Russians often write е instead of ё, even when ё is correct. So you may sometimes see:

• ребенка

But it is still pronounced ребёнка.

For learners, it is helpful to remember the dictionary form with ё, because it tells you both the pronunciation and the stress.

Yes, in some contexts that translation works, but sat the child down on the chair or seated the child on the chair is usually more precise.

Why?

Because посадить strongly suggests placing someone into a sitting position, not just moving them physically.

So:

• put the child on the chair = possible, but a bit less exact
• sat the child down on the chair = very close
• seated the child on the chair = also accurate, slightly more formal

The best English translation depends on style, but the Russian verb specifically carries the idea of making the child sit.