Thức ăn thừa bị lẫn trong túi rác.

Thức ăn thừa means leftover food.

• thức ăn = food
• thừa = leftover, extra, remaining

In Vietnamese, it is normal to put thừa after the noun, so thức ăn thừa is literally food leftover.

You may also hear:

• đồ ăn thừa = leftover food
• cơm thừa canh cặn = leftovers in a more idiomatic sense

In Vietnamese, descriptive words often come after the noun, not before it as in English.

So:

• thức ăn thừa = leftover food
• nhà lớn = big house
• áo đỏ = red shirt

This is a very common word order pattern in Vietnamese:

• noun + description

Bị often marks a situation that is passive and usually unfavorable, unwanted, or accidental.

So in Thức ăn thừa bị lẫn trong túi rác, bị suggests that the leftover food ended up mixed in the trash bag in a way that is not desirable.

Compare:

• bị lẫn = got mixed in / was mixed in unintentionally
• without bị, the sentence may sound more neutral or less clearly passive

English learners often think of bị as similar to was in a passive sentence, but it also carries the feeling that something bad or unfortunate happened.

Here, lẫn means something like to be mixed in, to get mixed together, or to be among other things by mistake.

In this sentence, it suggests the leftover food is mixed in with the garbage rather than being separately placed.

Common related uses:

• lẫn với = mixed with
• nhầm lẫn = confusion / mistake
• tiền giả lẫn với tiền thật = fake money mixed with real money

So bị lẫn trong túi rác gives the idea that the food got mixed into the trash bag.

Both are possible, but they focus on slightly different things.

• lẫn với emphasizes what it is mixed with

  • Thức ăn thừa bị lẫn với rác.
  • The leftover food got mixed with the trash.

• lẫn trong emphasizes the place or group where it is found

  • Thức ăn thừa bị lẫn trong túi rác.
  • The leftover food was mixed in the trash bag.

So trong túi rác focuses on the fact that it ended up inside the trash bag.

Trong means in / inside, so it describes a state or location.

• trong túi rác = in the trash bag

Vào usually suggests movement into something.

Compare:

• bị lẫn trong túi rác = was mixed in the trash bag
• bỏ vào túi rác = put into the trash bag

So the sentence is describing where the food ended up, not the action of putting it there.

Túi rác means trash bag or garbage bag.

• túi = bag
• rác = trash, garbage

There is no classifier here because the sentence is just naming the object in a general way.

Vietnamese classifiers are often used when:

• counting things
• specifying a particular item

For example:

• một túi rác = a trash bag
• cái túi rác này = this trash bag

But in the sentence trong túi rác, no classifier is needed.

The sentence follows this pattern:

Subject + bị + verb + location

So:

• Thức ăn thừa = subject
• bị = passive/unfavorable marker
• lẫn = mixed in
• trong túi rác = location

A natural breakdown is:

Thức ăn thừa / bị lẫn / trong túi rác.
Leftover food / got mixed in / in the trash bag.

This is a very common Vietnamese structure.

Vietnamese nouns usually do not mark singular vs. plural the way English does.

So thức ăn thừa could mean:

• leftover food
• the leftover food
• some leftover food

The exact number depends on context.

If the speaker wants to make plurality clearer, they might add words like:

• những for plural countable things
• quantity expressions

But with something like food, Vietnamese often leaves it unmarked.

Yes, very likely.

The combination bị + lẫn strongly suggests:

• it was not intentional
• it is probably undesirable
• it may have happened by mistake

So the sentence does not sound like someone deliberately sorted leftover food into the trash bag. It sounds more like the food ended up there unintentionally or was mistakenly mixed in.

Yes, you could, but it changes the nuance.

• Thức ăn thừa bị lẫn trong túi rác
  = The leftover food got mixed into the trash bag, with an accidental/unfortunate feeling.

• Thức ăn thừa lẫn trong túi rác
  = The leftover food is mixed in the trash bag. This sounds a bit more like a plain description.

So bị adds the sense that something unfortunate happened.

Yes. Bỏ lẫn is also very common and can sound even more explicit about something being put in by mistake.

For example:

• Thức ăn thừa bị bỏ lẫn vào túi rác.

This suggests someone accidentally threw or placed the leftover food into the trash bag.

Compared with:

• bị lẫn = got mixed in
• bị bỏ lẫn = was mistakenly put/thrown in together with something else

So the original sentence is natural, but bỏ lẫn is another useful expression to know.