Learn Hardcore Chinese: wǒmen yòng guo de sùliào dàizi yīnggāi fàng dào huíshōu de lājītǒng lǐ. - Used plastic bags should be put into the recycling trash can.

If you just said 我们用的塑料袋子, it would still be understandable, but it sounds less specific about the bags being already used in the past. 用过的 emphasizes that the using is a past, completed experience relevant to now (they are used bags, not new ones).

What is the function of 的 in 用过的塑料袋子?

Here 的 links a verb phrase to a noun, turning the verb phrase into an adjective‑like modifier:

我们用过的 → “that we have used” (relative clause)
塑料袋子 → “plastic bags”

So 我们用过的塑料袋子 literally is “the plastic bags [that we have used]”.

Structure:

[Subject + Verb (+ Aspect)] + 的 + Noun
我们用过的 + 塑料袋子
This is the normal way in Chinese to say “the N that S V-ed”.

Why does the whole modifier 我们用过的 come before 塑料袋子, instead of after it like in English?

Chinese puts almost all modifiers before the noun.

English:

“the plastic bags that we have used”

Chinese order is reversed:

我们用过的塑料袋子
literally: “we have‑used‑的 plastic bags”

So when you see a long chunk ending in 的 before a noun, it is usually a relative clause or descriptive phrase modifying that noun.

Is 袋子 necessary after 塑料? Could I just say 塑料袋?

You can say 塑料袋 or 塑料袋子. Both mean “plastic bag”.

袋 means “bag” (more concise, often used in writing or in compounds).
袋子 is the same word with 子, which often makes things sound a bit more colloquial or concrete.

So:

塑料袋 – perfectly fine, common
塑料袋子 – also common, very natural in spoken Chinese

The sentence would still be correct as 我们用过的塑料袋应该放到回收的垃圾桶里.

What does 应该 express here? Is it like “must” or “should”?

应该 means “should / ought to”, expressing advice, expectation, or moral obligation, but usually not absolute necessity.

In this sentence:

…应该放到回收的垃圾桶里。
“(They) should be put in the recycling bin.”

If you wanted something stronger, like “must”, you might use:

必须 (must, have to, very strong requirement)
要 (have to, need to, depending on context can be softer or strong)

But 应该 is good for talking about what is environmentally responsible / recommended.

What does 放到 mean, and why do we need 到 after 放?

放 means “to put / to place”.
到 here is a direction/result complement, indicating “to (a place)” or “reaching” a location.

放到回收的垃圾桶里 literally means “put (them) so that they arrive at / end up in the recycling bin”.

Without 到, 放回收的垃圾桶里 is still understandable, but 放到 is more standard and clearly shows movement to a destination.
You could also hear 放进回收的垃圾桶里, where 进 specifically emphasizes entering / going inside.

Why is there another 的 in 回收的垃圾桶?

Here 回收的 is again an adjective‑like modifier formed with 的:

回收 – “recycle / recycling” (verb or noun)
回收的垃圾桶 – literally “recycling garbage can”

It means a trash can used for recycling, i.e. a recycling bin.

Grammatically, it is the same pattern as before:

[Modifier] + 的 + Noun
回收的 + 垃圾桶 → “recycling + 的 + trash can”

What is the role of 里 in 垃圾桶里? Can it be left out?

里 means “inside / in”.

垃圾桶 = trash can
垃圾桶里 = inside the trash can

So 放到回收的垃圾桶里 = “put (them) into the recycling bin.”

If you drop 里 and say 放到回收的垃圾桶, it sounds incomplete or less natural, because 到 expects a kind of destination, and with containers we normally specify 里 to show “into / inside”.

Why isn’t there a plural marker for “bags” in 塑料袋子?

Chinese usually does not mark plural on nouns the way English does.

塑料袋子 can mean:

“a plastic bag”
“plastic bags”

Number is determined by context or by adding a measure word if needed:

一个塑料袋子 – one plastic bag
一些塑料袋子 – some plastic bags
很多塑料袋子 – many plastic bags

In this sentence, because we are talking about what to do in general with used plastic bags, the bare noun 塑料袋子 naturally implies the plural.

Why is it 我们用过的塑料袋子 and not 我们的用过塑料袋子?

我们的 means “our”, a possessive. That structure would be wrong here.

We want “the plastic bags that we have used”, not “our used plastic bags” (as a simple possession phrase).

Chinese relative‑clause pattern is:

[Subject + Verb (+ Aspect)] + 的 + Noun
我们用过的 + 塑料袋子

If you said 我们的用过塑料袋子, it would sound ungrammatical because:

我们的 should directly modify a noun,
but 用过 is a verb phrase, not replacing the role of 的.

You need 我们用过的 as one chunk modifying 塑料袋子.

Could we say 我们用了的塑料袋子 instead of 我们用过的塑料袋子?

You might occasionally see forms like 用了的, but here 用过的 is far more natural and standard.

Nuance:

过 focuses on having the experience or the state of “having done it before / already done”.
了 is more about completion or change of a specific event.

For “the plastic bags that we have used (already, as opposed to new bags)”, 用过的 captures this state better.
用了的塑料袋子 would sound awkward or overly marked to most native speakers in this context.

Could the sentence start without 我们, like 用过的塑料袋子应该放到回收的垃圾桶里?

Yes.

用过的塑料袋子应该放到回收的垃圾桶里。
This is still natural and means roughly “Used plastic bags should be put in the recycling bin.”

By dropping 我们, you remove the specific subject “we” and make the statement more general: about anyone’s used plastic bags.

With 我们, it is slightly more specific: “The plastic bags that we have used should be put into the recycling bin.”

Can I rewrite the sentence using 把, like 我们应该把用过的塑料袋子放到回收的垃圾桶里?

Yes, that is a very natural alternative.

我们应该把用过的塑料袋子放到回收的垃圾桶里。

Here:

把 + Object + Verb + (Complement)
把用过的塑料袋子放到回收的垃圾桶里

This emphasizes what happens to the object (the used plastic bags). Both versions are correct:

我们用过的塑料袋子应该放到回收的垃圾桶里。
– Topic is the bags themselves.
我们应该把用过的塑料袋子放到回收的垃圾桶里。
– Topic is what we should do with the bags.

The meaning is very close; it’s more about focus and style.