hanami no tame ni, bentoubako ni onigiri wo zyukko tumemasita.

It means for the purpose of cherry-blossom viewing or for the hanami outing.

Here, ために expresses purpose. So the sentence is saying the lunch box was packed for hanami.

A useful pattern is:

• noun + のために = for the sake/purpose of noun
• dictionary-form verb + ために = in order to verb

So:

• 花見のために = for hanami
• 日本へ行くために = in order to go to Japan

Because 花見 is a noun, and when a noun modifies ため, you usually use の.

So:

• 花見のために = for hanami

Compare that with a verb:

• おにぎりを作るために = in order to make rice balls

So the rule is:

• noun + のために
• verb + ために

That is why 花見ために would be incorrect.

In this sentence, に marks the destination/container into which something is packed.

• 弁当箱に = into the lunch box
• おにぎりを = rice balls
• 詰めました = packed/filled

So the structure is basically:

put X into Y

In Japanese, that often becomes:

Y に X を 詰める

So here:

• 弁当箱に = into the lunch box
• おにぎりを = rice balls

Because おにぎり is the thing being packed, so it is the direct object of 詰めました.

• おにぎりを詰めました = packed rice balls

Meanwhile, 弁当箱に is not the direct object. It is the place/container the rice balls go into.

So the roles are:

• 弁当箱に = into the lunch box
• おにぎりを = rice balls
• 詰めました = packed

Japanese counters can often appear in a few places, but おにぎりを十個詰めました is very natural.

Here, 十個 tells you how many rice balls were packed.

A very common pattern is:

• noun + を + number-counter + verb

So:

• おにぎりを十個詰めました = packed ten rice balls

You could also see things like:

• 十個のおにぎりを詰めました

This also means packed ten rice balls, but it feels a little more like ten rice balls is being treated as one noun phrase.

Both are possible, but the original sentence is very standard and natural.

個 is a common counter for small, compact objects.

Rice balls are often counted with 個, so:

• おにぎり十個 = ten rice balls

A learner may wonder about つ, since つ is a general native Japanese counter. While 十個 is very natural here, つ does not go up to ten in the same way. The native つ counting system is:

• ひとつ, ふたつ, みっつ ... ここのつ

After 9, it no longer works the same way, so for 10 and above, counters like 個 are very useful.

So 十個 is exactly what you would expect here.

Not exactly. 詰める means more like:

• pack
• stuff
• fill
• put in closely

So 弁当箱におにぎりを十個詰めました suggests not just placing them casually, but packing the lunch box with ten rice balls.

This verb is especially natural for things going into:

• boxes
• bags
• containers
• lunch boxes

So compared with a more general verb like 入れました:

• 入れました = put in
• 詰めました = packed in / filled in

Japanese often omits the subject when it is understood from context.

So even though English might require something like I packed or we packed, Japanese often leaves that unsaid.

In this sentence, the subject could be:

• I
• we
• someone already being talked about

Without context, the sentence simply means that someone packed ten rice balls into a lunch box for hanami.

This omission is very normal in Japanese.

Because the action is being presented as completed.

• 詰めました = packed
• 詰めます = pack / will pack

So this sentence describes something that has already been done.

The ました ending also makes it polite. It is the polite past form of 詰める.

So the sentence is both:

• past
• polite

No. In normal Japanese writing, it would usually be written without spaces:

花見のために、弁当箱におにぎりを十個詰めました。

Spaces are often added in teaching materials to help learners see the parts more clearly.

So the spaced version is for readability, but the standard written form does not use those spaces.

Sometimes learners think of replacing it with 花見に, but the meaning changes.

• 花見のために = for the purpose of hanami
• 花見に often means to hanami or for hanami depending on context, but it does not emphasize purpose in the same clear way here

In this sentence, 花見のために clearly tells you why the speaker packed the rice balls.

So it is a good choice when you want to stress the purpose: the lunch box was prepared for the hanami outing.