asita ha kouen de hanami wo sinagara, bentoubako ni ireta onigiri wo taberu tumori desu.

は marks 明日 as the topic: as for tomorrow.

So 明日は does not just mean tomorrow in a neutral dictionary sense; it sets the time frame for the whole sentence. In English, we often just say Tomorrow, ..., but in Japanese, は is very common after time expressions when they are being presented as the topic or contrastive frame.

Compare:

• 明日、行きます。 = I’m going tomorrow.
• 明日は、行きます。 = As for tomorrow, I’m going.

The second one can also carry a slight feeling of contrast depending on context.

Because で marks the place where an action happens.

Here, the actions are:

• 花見をする = do flower viewing / enjoy cherry blossoms
• 食べる = eat

Those actions take place at the park, so 公園で is correct.

Use に for a destination, existence, or a point of arrival, for example:

• 公園に行く = go to the park
• 公園にいる = be in the park

But:

• 公園で遊ぶ = play at the park
• 公園で食べる = eat at the park

花見 is a noun meaning flower viewing or cherry-blossom viewing. In Japanese, many nouns can combine with する to make a verb-like expression.

So:

• 花見 = flower viewing
• 花見をする = to do flower viewing / to go enjoy the blossoms

The particle を marks 花見 as the thing being done with する.

This pattern is very common:

• 勉強をする = study
• 運動をする = exercise
• 買い物をする = shop / do shopping

ながら means while doing or as one does.

It connects two actions done by the same person at roughly the same time.

Here:

• 花見をしながら
• おにぎりを食べる

So the speaker plans to eat rice balls while enjoying the flower viewing.

A key point: with ながら, the action before ながら is often the background or accompanying action, while the action after it is often the main action.

So this sentence naturally feels like:

• While looking at the blossoms, I plan to eat rice balls.

Because ながら attaches to the verb stem.

For する, the stem is し.

So:

• する → しながら
• 食べる → 食べながら
• 見る → 見ながら
• 飲む → 飲みながら

This is the standard formation.

This is a noun-modifying clause.

Japanese often puts a whole clause before a noun to describe it.

So:

• 弁当箱に入れた = put into a lunch box
• おにぎり = rice balls

Together:

• 弁当箱に入れたおにぎり = the rice balls that were put into a lunch box

In English, we often need a relative pronoun like that or which:

• the rice balls that I put into a lunch box

In Japanese, there is no relative pronoun here. The clause simply comes before the noun.

Because 入れた does not mean the entire sentence is in the past. It shows that the putting into the lunch box is completed before the rice balls are eaten.

Inside a noun-modifying clause, the tense is relative to the noun and situation, not necessarily to the present moment of speaking.

So 弁当箱に入れたおにぎり means:

• rice balls that have been put into a lunch box
• rice balls I will have put into a lunch box before eating them

Even though the plan is for tomorrow, the rice balls will already be in the lunch box at the time of eating, so 入れた makes sense.

Here, に marks the destination or container into which something is put.

• 弁当箱に入れる = put something into a lunch box

So:

• 弁当箱 = lunch box
• に = into
• 入れた = put in / inserted

This is similar to:

• かばんに本を入れる = put a book into a bag
• 箱にお菓子を入れる = put sweets into a box

Because つもりです attaches to the plain form of a verb, not the ます-form.

So:

• 食べるつもりです = intend to eat
• 行くつもりです = intend to go
• しないつもりです = intend not to do

Not:

• 食べますつもりです ✘

This is a very important pattern:

• dictionary/plain form + つもりです

つもりです expresses intention or plan.

So 食べるつもりです means something like:

• I intend to eat
• I plan to eat

It is not just a neutral future statement. It emphasizes the speaker’s decision or expectation.

Compare:

• 食べます。 = I will eat / I eat
• 食べるつもりです。 = I intend to eat / I’m planning to eat

The second sounds more like a personal plan.

Yes, the subject is omitted, and it is very naturally understood as I from context.

Japanese often leaves out subjects when they are clear. In a sentence ending with つもりです, the speaker is usually talking about their own intention unless the context says otherwise.

So this sentence naturally means:

• I plan to eat...

If you explicitly said the subject, it might be:

• 私は明日は公園で花見をしながら、弁当箱に入れたおにぎりを食べるつもりです。

But this is often unnecessary.

Yes. Each particle marks the role of the word before it.

Breakdown:

• 明日 は — tomorrow as the topic
• 公園 で — at the park, where the action happens
• 花見 を — flower viewing as the thing done with する
• 弁当箱 に — into the lunch box
• おにぎり を — rice balls as the direct object of 食べる

Particles are one of the main ways Japanese shows grammatical relationships, since word order is more flexible than in English.

To some extent, yes. Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

For example, these can still be understandable:

• 明日は弁当箱に入れたおにぎりを、公園で花見をしながら食べるつもりです。
• 明日は公園で、弁当箱に入れたおにぎりを花見をしながら食べるつもりです。

However, the original sentence is quite natural because it presents the information in a smooth order:

1. time
2. place
3. accompanying action
4. object
5. main verb + intention

So while word order can move around, not every rearrangement sounds equally natural.

Not a strict grammatical one. It mainly helps readability and shows a small pause.

The sentence could be written without it:

• 明日は公園で花見をしながら弁当箱に入れたおにぎりを食べるつもりです。

That is still correct. The comma just makes it easier to read by separating the while doing X part from the rest.

Not strictly from grammar alone.

Grammatically, it just means:

• rice balls that were put into a lunch box

In context, many listeners would naturally assume the speaker prepared or packed them, but the phrase itself does not explicitly say who put them there.

If the speaker wanted to make I put them in clearer, context usually does that. Japanese often leaves that kind of information implicit.