imouto ha buranko ni nottaato, sunaba de tomodati to asobimasita.

は marks 妹 as the topic of the sentence.

So 妹は means something like:

• As for my younger sister...
• My younger sister...

It does not exactly mean the same thing as an English subject marker. It tells the listener what the sentence is about. In this sentence, everything that follows describes what the younger sister did.

Japanese often leaves out words like my, your, or the when they are understood from context.

So 妹 literally just means younger sister, but in a normal sentence it often naturally refers to my younger sister unless the context says otherwise.

This is very common in Japanese:

• 母 can mean my mother
• 友達 can mean a friend or friends
• Japanese relies heavily on context instead of explicit possessives and articles

ブランコ means swing (the playground kind).

It is written in katakana because it is a loanword. Many borrowed words in Japanese are written in katakana.

So:

• ブランコ = swing
• katakana often marks foreign-origin vocabulary

Even though the original source of the word may not be English, for a learner the important point is that katakana is commonly used for this kind of word.

The verb 乗る（のる） usually takes に, not を, when it means to ride or to get on something.

So:

• ブランコに乗る = to ride a swing
• 電車に乗る = to get on / ride a train
• 自転車に乗る = to ride a bicycle

The particle に marks the thing you get onto or ride.

This is different from English, where we think of ride as taking a direct object. In Japanese, you should learn 乗る together with に.

乗ったあと means after riding or after she rode.

It is made from:

• 乗った = past plain form of 乗る
• あと = after

So the structure is:

• verb in plain past + あと = after doing something

Examples:

• 食べたあと = after eating
• 帰ったあと = after going home
• 勉強したあと = after studying

In this sentence, ブランコに乗ったあと means after riding the swing.

Before あと, Japanese normally uses the plain past form to show that one action is completed before the next one.

So:

• 乗ったあと = after riding
• not 乗るあと

This pattern shows sequence clearly:

1. she rode the swing
2. then she played in the sandbox with her friend

It is a very common pattern for saying after doing X, ...

で marks the place where an action happens.

So:

• 砂場で遊びました = played in the sandbox
• literally, at/in the sandbox, played

Compare:

• 学校で勉強する = study at school
• 公園で遊ぶ = play in the park

This is different from に, which often marks:

• destination
• time
• existence
• something you get onto with verbs like 乗る

Here, since playing happens in the sandbox, で is the correct particle.

Here と means with.

So:

• 友達と遊びました = played with a friend / with friends

This と marks a companion, the person someone does something with.

Examples:

• 母と話した = talked with my mother
• 先生と行った = went with the teacher

Be careful: と can also mean and when joining nouns, but here it means with because it follows a person and connects to the action 遊びました.

By itself, 友達 can mean either friend or friends, depending on context.

In this sentence, English might translate it as:

• with a friend
• with friends

Both are possible unless the larger context makes it clear.

Japanese nouns usually do not change form for singular vs. plural the way English nouns do. Context tells you which meaning is intended.

遊びました is the polite past form of 遊ぶ.

• dictionary form: 遊ぶ = to play
• polite non-past: 遊びます
• polite past: 遊びました

It is polite because it uses the -ます style, which is common in textbooks and everyday respectful speech.

It is past tense because the whole sentence describes completed actions:

• she rode the swing
• then she played in the sandbox

In Japanese, the main verb usually comes at the end of the sentence.

Earlier actions can appear before it in modifying forms, like:

• 乗ったあと = after riding

So the sentence is structured more like:

• As for my younger sister, after riding the swing, in the sandbox with a friend, played.

That sounds unnatural in English, but it is normal in Japanese. Japanese often builds context first and gives the main action at the end.

Yes, Japanese word order is somewhat flexible, as long as the particles make the roles clear.

For example, parts like 砂場で and 友達と could sometimes move around:

• 妹はブランコに乗ったあと、友達と砂場で遊びました。

This still means the same thing.

However, the verb still tends to come at the end, and some orders sound more natural than others. The original sentence is very natural and easy to follow.

The comma helps separate the after riding the swing part from the main action that follows.

So it makes the sentence easier to read:

• ブランコに乗ったあと、
• then the main event: 砂場で友達と遊びました

Japanese commas are often used more flexibly than English commas. In this sentence, the comma is natural and helpful, though punctuation in Japanese can sometimes vary depending on style.