Потом она положила ракушку на песочный замок и спрятала ведёрко под шезлонг.

Because the subject is она (she), and the sentence is in the past tense.

In Russian past tense:

• masculine: -л
• feminine: -ла
• neuter: -ло
• plural: -ли

So:

• она положила = she put
• она спрятала = she hid

If the subject were он, it would be:

• он положил
• он спрятал

Both положить and спрятать are perfective verbs here because the sentence describes completed actions in a sequence:

1. she put the shell on the sandcastle
2. she hid the bucket under the deck chair

Perfective verbs are very common in storytelling when you are talking about single, finished events.

Compare:

• класть / положить = to put, place
  
  • класть = imperfective
  • положить = perfective
• прятать / спрятать = to hide
  
  • прятать = imperfective
  • спрятать = perfective

So this sentence sounds like a completed episode in a story, not an ongoing process.

Because it is the direct object of положила, so it goes into the accusative case.

The noun ракушка is feminine, and feminine nouns ending in -а usually change like this in the accusative singular:

• nominative: ракушка
• accusative: ракушку

So:

• положила что? → ракушку

This is a very common pattern:

• книга → книгу
• чашка → чашку
• игрушка → игрушку

It is in the accusative, but for many neuter inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: ведёрко
• accusative: ведёрко

That is why you do not see a spelling change.

It is still the direct object of спрятала:

• спрятала что? → ведёрко

This is normal for neuter inanimate nouns:

• яблоко
• письмо
• ведро
• ведёрко

Because на can take different cases depending on meaning.

Here it means movement onto something:

• положила на песочный замок = she put it onto the sandcastle

With movement toward a destination, на takes the accusative:

• на стол
• на пол
• на песочный замок

If you were describing location, not movement, you would usually use the prepositional:

• на песочном замке = on the sandcastle

So the contrast is:

• Она положила ракушку на песочный замок.
  She put the shell onto the sandcastle.
• Ракушка лежала на песочном замке.
  The shell was lying on the sandcastle.

For the same reason: под changes case depending on whether you mean movement or location.

Here there is movement to a new place:

• спрятала ведёрко под шезлонг = hid the bucket under the deck chair

With movement, под takes the accusative.

If you were talking about location, it would usually take the instrumental:

• под шезлонгом = under the deck chair

Compare:

• Она спрятала ведёрко под шезлонг.
  She hid the bucket under the deck chair.
• Ведёрко было под шезлонгом.
  The bucket was under the deck chair.

Yes. Russian often expresses this idea as an adjective + noun:

• песочный = sandy / made of sand / relating to sand
• замок = castle

So песочный замок literally means sand castle, which is the normal Russian expression for sandcastle.

A learner may notice that песочный can also mean things like sand-based or for sand, depending on context. Here the meaning is clearly sandcastle.

Both can mean then / afterwards / after that, but потом is very common and natural in everyday speech.

In this sentence:

• Потом = Then / After that

It helps move the story forward.

Very roughly:

• потом = common, conversational, neutral
• затем = also correct, sometimes a little more formal or literary depending on context

So Потом она положила... sounds very natural in a simple narrative.

Yes, Russian word order is flexible. The version in the sentence is neutral and natural for storytelling.

Current order:

• Потом она положила ракушку... и спрятала ведёрко...

This flows like:

1. time marker
2. subject
3. actions

You could rearrange parts for emphasis, for example:

• Она потом положила ракушку...
• Ракушку она потом положила на песочный замок...

But those versions may shift the emphasis or sound less neutral in a beginner context.

So the given order is a good standard narrative order.

Because in Russian, just like in English, you do not need to repeat the subject if it is clearly the same subject.

So:

• Потом она положила ракушку на песочный замок и спрятала ведёрко под шезлонг.

means:

• Then she put the shell on the sandcastle and hid the bucket under the deck chair.

The subject она applies to both verbs:

• положила
• спрятала

If Russian repeated она, it would usually add emphasis or sound stylistically different:

• ...и она спрятала ведёрко...

Ведёрко is a diminutive form of ведро (bucket / pail).

So:

• ведро = bucket
• ведёрко = little bucket / buckety little thing / cute bucket

In many contexts, especially with children, beaches, toys, or affectionate description, Russian often uses diminutives very naturally.

Here ведёрко sounds especially fitting because it is probably a small child’s beach bucket.

Also note the spelling:

• ё in ведёрко

That tells you the stressed sound is yo:

• vye-DYOR-ka

Yes, in everyday Russian ракушка is the usual common word for a shell or seashell, and it has a somewhat diminutive-looking form.

The more formal/basic noun is:

• раковина

But раковина can also mean sink in modern Russian, so in beach contexts ракушка is the very natural word for seashell.

So in this sentence:

• ракушка = shell / seashell

It sounds normal and idiomatic.

It is in the accusative singular because it follows на in the sense of movement onto something.

• nominative: песочный замок
• accusative: песочный замок

For an inanimate masculine noun like замок, nominative and accusative singular are the same in form.

So even though the ending does not change visibly, the case is still accusative.

It is also in the accusative singular, because под here means movement to a place:

• под шезлонг = under the deck chair

Like замок, шезлонг is an inanimate masculine noun, so nominative and accusative singular look the same:

• nominative: шезлонг
• accusative: шезлонг

If it were location instead of movement, you would get instrumental:

• под шезлонгом

Шезлонг means deck chair or lounge chair.

Yes, it is a borrowing, originally from French chaise longue, though Russian took it in its own adapted form:

• шезлонг

It is a normal Russian word now, often used for beach or poolside reclining chairs.

So:

• под шезлонг = under the deck chair

Usually no, or at least not as naturally.

Russian often chooses different verbs depending on how an object is placed:

• положить = to lay/put something down, often horizontally or generally place it
• поставить = to set/stand something upright

A shell is not something you normally stand upright, so:

• положила ракушку sounds natural
• поставила ракушку would sound odd in most contexts

So положила is the right choice here.

Then Russian would probably use verbs of location and different cases after на and под.

For example:

• Ракушка лежала на песочном замке, а ведёрко было под шезлонгом.

Notice the changes:

• на песочном замке = location, so prepositional
• под шезлонгом = location, so instrumental

That is a very useful contrast with the original sentence, which describes movement:

• на песочный замок
• под шезлонг