Sako enhavas tiom da kartono kiom da plasto.

It is the normal Esperanto pattern for comparing quantities.

• tiom = that much / so much / as much
• kiom = how much, or in this kind of pairing, as much as

So tiom da kartono kiom da plasto means as much cardboard as plastic.

A useful way to remember it is:

• tiom ... kiom ... = equal amount
• English often translates this as as much ... as ...

Because da is used after words of quantity when they are followed by the thing being measured.

So:

• tiom da kartono = that much cardboard
• kiom da plasto = as much plastic

This is the same da you see in expressions like:

• multe da akvo = a lot of water
• iom da tempo = some time
• kilogramo da rizo = a kilogram of rice

In this sentence, both halves of the comparison talk about an amount, so both need da.

Because they are being treated as substances or materials, not as separate countable items.

In Esperanto, material nouns often work like English mass nouns:

• akvo = water
• ligno = wood
• papero = paper
• plasto = plastic
• kartono = cardboard

So kartono and plasto stay singular when you mean the material in general.

If you used the plural, it would usually suggest different kinds or separate individual items, not just an amount of the material.

Because after da, the noun normally does not take the accusative ending.

The whole quantity expression functions as the object:

• tiom da kartono kiom da plasto

So Esperanto does not normally say:

• tiom da kartonon
• kiom da plaston

Those would sound wrong in standard usage.

Compare:

• La sako enhavas kartonon. = The bag contains cardboard.
• La sako enhavas multe da kartono. = The bag contains a lot of cardboard.

In the second sentence, kartono stays without -n because it is part of a da phrase.

Not in this sentence.

Yes, kiom can be a question word:

• Kiom da akvo? = How much water?

But here it is not asking a question. It is part of a matching pair:

• tiom ... kiom ...

In that pair, kiom means something like as much as.

So here it is a comparison word, not an actual question.

Because this sentence compares quantity, not manner or quality.

Use kiom when you are talking about how much / how many.

Use kiel when you are talking about how / in what way / like, or in comparisons of qualities.

For example:

• Li havas tiom da mono kiom mi. = He has as much money as I do.
• Li estas tiel alta kiel mi. = He is as tall as I am.

So:

• tiom ... kiom ... = comparison of amount
• tiel ... kiel ... = comparison of degree, manner, or quality

Since kartono and plasto are quantities of material, kiom is the right choice.

Because Esperanto has no indefinite article like English a or an.

It only has the definite article la.

So:

• sako can mean a bag or just bag in a general/example sentence
• la sako means the bag, a specific bag already known from context

In isolated teaching examples, Esperanto often leaves out la unless the noun is clearly specific.

So Sako enhavas ... is normal as an example sentence.

Yes, but tiom da is the more normal and compact form here.

Compare:

• tiom da kartono = that much cardboard / as much cardboard
• tiom multe da kartono = that very much cardboard, with extra emphasis

The version with multe is possible, but it sounds heavier and usually adds emphasis. In a simple neutral comparison, tiom da ... kiom da ... is the usual choice.

Because enhavi means to contain, which fits a bag very naturally.

• havi = to have
• enhavi = to contain

A bag can in some contexts have things, but contain is more precise because it specifically refers to what is inside it.

So:

• La sako havas truon. = The bag has a hole.
• La sako enhavas plaston. = The bag contains plastic.

In this sentence, enhavas is the most exact verb.

They break down like this:

• Sako = the subject
• enhavas = the verb
• tiom da kartono kiom da plasto = the object phrase, expressing the amount contained

So the basic structure is:

• [subject] [verb] [quantity expression]

That can help you see that the sentence is not doing anything unusual with word order. Most of the complexity comes from the quantity comparison tiom ... kiom ..., not from the sentence structure itself.