Когда погремушка падает на пол, мама моет её и снова кладёт рядом с кроваткой.

Because the sentence describes a habitual / repeated situation, not one single event.

• Когда погремушка падает на пол = whenever / when the rattle falls on the floor
• мама моет её и снова кладёт... = mom washes it and puts it back again

In Russian, the present tense is very commonly used for things that happen regularly.

If you wanted a one-time future event, Russian would usually use perfective future forms instead:

• Когда погремушка упадёт на пол, мама вымоет её и снова положит рядом с кроваткой.

So the original sentence is more like a routine: this is what mom does whenever it happens.

These forms show different meanings:

• падает = is falling / falls, imperfective, present
• упадёт = will fall, perfective, future
• упала = fell, perfective, past

Here, падает is used because the sentence is about a repeated action. The rattle falls from time to time, and each time mom responds.

So:

• Когда погремушка падает на пол... = whenever the rattle falls onto the floor...
• Когда погремушка упадёт на пол... = when the rattle falls onto the floor (one future occasion)...

This is about aspect.

The sentence uses imperfective verbs:

• моет from мыть
• кладёт from класть

Imperfective verbs are natural when talking about:

• repeated actions
• habits
• general routines
• actions viewed as a process

That fits this sentence perfectly: every time the rattle falls, mom washes it and puts it back.

If you said:

• вымоет
• положит

that would sound more like one completed future action on one particular occasion.

So the contrast is roughly:

• мама моет её и снова кладёт... = this is what she does regularly
• мама вымоет её и снова положит... = she will wash it and put it back (one time)

Because Russian uses different cases depending on whether you mean movement toward a place or location in a place.

With на:

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

So:

• падает на пол = falls onto the floor → motion → accusative
• лежит на полу = lies on the floor → location → prepositional

That is why the sentence has:

• на пол (accusative)

not:

• на полу (prepositional)

Because её is the object form of она.

• она = she
• её = her / it

In this sentence, the pronoun refers back to погремушка, which is a feminine noun. It is the thing being washed, so it is a direct object:

• мама моет её = mom washes it

You cannot use она there, because она is a subject form.

Compare:

• Она падает. = It/she falls.
• Мама моет её. = Mom washes it/her.

Because in the present tense, Russian verbs agree only in:

• person
• number

They do not agree in gender.

So both masculine and feminine singular subjects take the same kind of 3rd person singular present form:

• мама моет
• папа моет
• погремушка падает

Gender shows up clearly in the past tense, not the present:

• мама мыла
• папа мыл
• погремушка упала

So even though мама and погремушка are feminine, the present-tense verbs stay in the normal 3rd person singular form.

Кроваткой is in the instrumental case.

That is because the expression рядом с is followed by the instrumental:

• рядом с кроваткой = next to the crib / little bed

So the pattern is:

• рядом с + instrumental

Examples:

• рядом с домом = next to the house
• рядом с мамой = next to mom
• рядом с кроваткой = next to the crib

The base form is:

• кроватка

and its instrumental singular is:

• кроваткой

Because the sentence uses кроватка, not кровать.

• кровать = bed
• кроватка = little bed, baby bed, crib

The ending changes because these are different nouns:

• кровать → instrumental кроватью
• кроватка → instrumental кроваткой

Here кроватка sounds natural because the context is a baby’s rattle, so a crib / little bed makes sense.

So the difference is not just grammar, but also nuance:

• кровать = neutral bed
• кроватка = more affectionate or baby-related little bed / crib

Снова means again.

Here it tells us that after washing the rattle, mom puts it back again near the crib:

• мама моет её и снова кладёт рядом с кроваткой

Its position is fairly flexible, but different placements can sound slightly different in emphasis.

For example:

• мама снова кладёт её рядом с кроваткой
• мама кладёт её снова рядом с кроваткой
• мама моет её и снова кладёт рядом с кроваткой

The original version is very natural because снова is placed right before the action it modifies: кладёт.

Because Когда погремушка падает на пол is a subordinate clause, and Russian normally separates it from the main clause with a comma.

So the structure is:

• Когда погремушка падает на пол, = subordinate clause
• мама моет её и снова кладёт рядом с кроваткой. = main clause

This is standard Russian punctuation.

English also usually uses a comma when a when-clause comes first:

• When the rattle falls on the floor, mom washes it...

The standard verb here is класть → кладёт.

This is the normal imperfective pair with положить:

• класть = to put, to place (imperfective)
• положить = to put, to place (perfective)

So:

• она кладёт = she is putting / she puts
• она положит = she will put

Many learners notice forms like ложить, but in standard Russian, ложить by itself is generally considered nonstandard. The normal standard pair is:

• класть / положить

That is why кладёт is the correct standard form here.

Yes. In many Russian texts, ё is often written as е.

So you may see:

• ее instead of её
• кладет instead of кладёт

But the pronunciation and dictionary form still involve ё:

• её sounds like ye-YO
• кладёт has -dyot / -dyot-type pronunciation, not plain -det

For learners, it is very useful to remember where ё really belongs, because it helps with:

• pronunciation
• stress
• recognizing word forms

Yes. Russian word order is more flexible than English, because the case endings show grammatical relationships.

The original order is very natural and neutral:

• Когда погремушка падает на пол, мама моет её и снова кладёт рядом с кроваткой.

But some variation is possible, for example:

• Когда погремушка падает на пол, её моет мама и снова кладёт рядом с кроваткой.
• Мама, когда погремушка падает на пол, моет её и снова кладёт рядом с кроваткой.

These alternatives may sound more marked or stylistically different. For a learner, the original sentence is the best neutral model to follow.

So yes, word order can change, but the given version is the most straightforward and natural.