asita no bentou ni, onigiri ya yasai wo tumete oku.

Yes. 明日 is a noun meaning tomorrow, and の links it to 弁当.

So:

• 明日 = tomorrow
• の = links one noun to another
• 弁当 = boxed lunch / bento

Together, 明日の弁当 means tomorrow’s bento or the lunch for tomorrow.

In Japanese, noun + の + noun is a very common pattern.

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

So in this sentence:

• 明日の弁当に = into tomorrow’s bento / for tomorrow’s lunchbox

Because the verb 詰める means to pack / stuff / fill, the thing being filled often takes に.

Compare:

• 箱に本を詰める = pack books into a box
• 弁当におにぎりや野菜を詰める = pack rice balls and vegetables into a bento

So に is showing where the food is being packed.

Because おにぎりや野菜 are the things being packed, and を marks the direct object of the verb.

Breakdown:

• おにぎりや野菜 = rice balls and vegetables and things like that
• を = direct object marker
• 詰めておく = pack in advance / put in beforehand

So を marks what is being packed.

や means something like and, but with the nuance of among other things or for example.

So:

• おにぎりや野菜 = rice balls, vegetables, and so on

If the sentence used と instead:

• おにぎりと野菜

that would sound more like a complete list: rice balls and vegetables.

So the difference is:

• A や B = A, B, etc. / A and B and other similar things
• A と B = A and B

In this sentence, や suggests these are examples of what will be packed, not necessarily the only items.

～ておく means doing something in advance, ahead of time, or leaving it done for later convenience.

So:

• 詰める = to pack / fill
• 詰めておく = to pack in advance / pack beforehand

In this sentence, the speaker is not just saying I pack rice balls and vegetables into tomorrow’s bento.
They are saying they will pack them ahead of time so it’s ready.

That preparation nuance is the key meaning of ～ておく.

They are related, but they focus on different things.

• ～ておく focuses on the action done in preparation.
• ～てある focuses on the resulting state after someone has done it.

For example:

• 弁当におにぎりを詰めておく
  = I’ll pack rice balls into the bento in advance.

• 弁当におにぎりが詰めてある
  = Rice balls have been packed into the bento.
  (They are in there already.)

So ～ておく is about preparing, while ～てある is about the prepared state.

Because Japanese often leaves out subjects when they are understood from context.

English usually needs a subject:

• I’ll pack...
• She packs...

Japanese often does not state it if it is obvious from the situation.

So this sentence could mean:

• I’ll pack rice balls and vegetables into tomorrow’s bento in advance
• We’ll pack...
• She’ll pack...

depending on context.

This omission is very normal in Japanese.

It can mean either, depending on context.

弁当 commonly refers to:

• a packed meal
• a boxed lunch
• sometimes the lunchbox meal as a whole

In this sentence, because the verb is 詰める (to pack into/fill), 弁当 strongly suggests the bento container or prepared lunch being filled with food.

So 明日の弁当に is most naturally understood as into tomorrow’s bento / into the lunch for tomorrow.

Japanese word order is flexible, but the basic tendency is:

• setting/context first
• object before the verb
• verb at the end

So here:

• 明日の弁当に = into tomorrow’s bento
• おにぎりや野菜を = rice balls and vegetables
• 詰めておく = pack in advance

Japanese usually saves the main verb for the end. That is one of the biggest differences from English.

A very literal ordering would be:

Into tomorrow’s bento, rice balls and vegetables [I] pack in advance.

That sounds strange in English, but it is normal in Japanese.

Not in the same way.

• 明日の弁当におにぎりや野菜を詰める
  = pack rice balls and vegetables into tomorrow’s bento

Here, 弁当 is the container/destination, so it takes に.

If you said:

• 明日の弁当を詰める

that would mean something more like:

• pack tomorrow’s bento

In that version, the entire bento is treated as the object being prepared.

So both can be natural, but they mean slightly different things:

• 弁当に～を詰める = put X into the bento
• 弁当を詰める = pack the bento itself

The comma is mainly a pause marker. It helps separate the destination phrase from the object phrase and makes the sentence easier to read.

So:

• 明日の弁当に、おにぎりや野菜を詰めておく。

feels like:

• As for tomorrow’s bento, I’ll pack rice balls, vegetables, and so on in advance.

The comma does not drastically change the grammar. It just improves clarity and rhythm.

Japanese nouns usually do not show singular/plural the way English does.

So 野菜 can mean:

• a vegetable
• vegetables

In this sentence, English would normally translate it as vegetables, because it is part of a food list:

• おにぎりや野菜 = rice balls and vegetables

The exact number is not important unless the speaker specifically says so.

Not only that. 詰める often means to pack, fill, or put things into a container, and it does not always imply forceful stuffing in everyday use.

For bentos, it is a very natural verb because you are putting food items into the bento box.

So here it simply means something like:

• pack into
• place into
• fill with

For a lunchbox, 詰める is more natural than a literal English word like stuff.