Όλα τα παιχνίδια μπαίνουν σε ένα ξύλινο σεντούκι όταν τελειώνει το παιχνίδι.

Because in Greek, all + plural noun normally takes the definite article.

So:

• όλα = all
• τα παιχνίδια = the toys / the games

Together, όλα τα παιχνίδια means all the toys/games.

This is very normal Greek structure:

• όλοι οι άνθρωποι = all the people
• όλες οι μέρες = all the days
• όλα τα βιβλία = all the books

English often drops the in this kind of phrase, but Greek usually keeps the article.

It can mean more than one thing, depending on context:

• toy
• game
• sometimes play / playtime

That is why this sentence may look confusing at first. In όλα τα παιχνίδια, the word may refer to toys. In το παιχνίδι, it may refer to the game or playtime.

So Greek is using the same noun in two slightly different but related senses.

Μπαίνουν is the 3rd person plural present of μπαίνω.

Basic meaning of μπαίνω:

• I go in
• I enter
• I get into

So:

• μπαίνω = I go in
• μπαίνεις = you go in
• μπαίνει = it/he/she goes in
• μπαίνουν = they go in

In this sentence, όλα τα παιχνίδια μπαίνουν... literally means all the toys/games go into...

English may prefer are put into, but Greek often uses μπαίνω naturally here.

Because Greek often expresses this idea from the point of view of the thing that moves, not necessarily the person doing the action.

So English might say:

• The toys are put in the chest.

Greek can naturally say:

• The toys go into the chest.

That does not mean the toys are moving by themselves in some magical way. It is just a normal Greek way to describe what happens to them.

A more explicitly passive-style wording is possible in Greek in some contexts, but μπαίνουν sounds very natural here.

Here it is best understood as into because the verb shows movement.

• σε can mean in, at, on, to, into, depending on context.
• The verb tells you a lot.

Compare:

• Είναι σε ένα κουτί. = It is in a box.
• Μπαίνει σε ένα κουτί. = It goes into a box.

So in this sentence, σε ένα ξύλινο σεντούκι means into a wooden chest or in a wooden chest, depending on how naturally you want to phrase it in English.

Because all three words match in gender, number, and case.

• ένα = a / one (neuter singular)
• ξύλινο = wooden (neuter singular)
• σεντούκι = chest (neuter singular noun)

The noun σεντούκι is neuter, so the adjective must also be neuter:

• ξύλινος = masculine
• ξύλινη = feminine
• ξύλινο = neuter

Since σεντούκι is neuter, Greek uses ξύλινο.

That is a very common learner question, especially with neuter nouns.

In Modern Greek, many neuter nominative and neuter accusative forms look the same.

In this sentence:

• όλα τα παιχνίδια = subject of μπαίνουν → nominative
• σε ένα ξύλινο σεντούκι = after σε → accusative
• το παιχνίδι = subject of τελειώνει → nominative

Even though παιχνίδι / παιχνίδια can look the same in nominative and accusative, their job in the sentence tells you what they are doing.

So you often identify the case by function, not only by form.

Because τελειώνει does not go with όλα τα παιχνίδια.

Its subject is το παιχνίδι, which is singular.

So the structure is:

• Όλα τα παιχνίδια μπαίνουν...
• όταν τελειώνει το παιχνίδι

In other words:

• the toys go into the chest
• when the game/playtime ends

So the plural subject belongs to the first verb, and the singular subject belongs to the second verb.

Because the sentence describes a general rule or habitual action.

It means something like:

• Whenever the game/playtime ends, all the toys go into a wooden chest.

Greek commonly uses the present tense for repeated or regular events.

So:

• μπαίνουν = go in / are put in
• τελειώνει = ends

If you were talking about one specific future occasion, Greek would often use a different form, for example:

• όταν τελειώσει το παιχνίδι = when the game finishes

So the present here signals something habitual or generally true.

Because Greek uses the definite article more often than English.

Το παιχνίδι can refer to:

• the game
• the play activity
• the usual playtime in that situation

English often prefers a more article-free expression like when playtime ends, but Greek naturally says όταν τελειώνει το παιχνίδι.

So even if English would not always use the, Greek often does.

Greek word order is fairly flexible.

This sentence could also be written as:

• Όταν τελειώνει το παιχνίδι, όλα τα παιχνίδια μπαίνουν σε ένα ξύλινο σεντούκι.

That version puts the when clause first.

The original order starts with όλα τα παιχνίδια, which helps make the toys/games the topic right away.

So the order can change, but the original sentence is perfectly natural.