Ni kolektis tiom da metalo kiom da vitro.

It is a comparison of quantity.

• tiom = that much / so much
• kiom = as much as / how much

So tiom da metalo kiom da vitro means as much metal as glass.

This pattern is very common in Esperanto:

• tiom ... kiom ... = as much/many ... as ...
• tiel ... kiel ... = as ... as for manner or degree
• same ... kiel is not the normal pattern here

In this sentence, the speaker is comparing the amount of metal collected with the amount of glass collected.

Because both parts of the comparison express an amount of a substance.

• tiom da metalo = that much metal
• kiom da vitro = as much glass

In Esperanto, da is used after words of quantity when you mean an amount of something:

• multe da akvo = a lot of water
• iom da tempo = some time
• tiom da metalo = that much metal

Since both halves of the comparison involve quantity, both need da.

Because here they are being treated as mass nouns, like metal and glass in English.

When you talk about a substance in general, Esperanto often uses the singular:

• akvo = water
• sablo = sand
• metalo = metal
• vitro = glass

So tiom da metalo does not mean one metal object. It means that amount of metal as a material.

If you wanted to talk about separate items, you would normally use a countable noun instead.

Because after da, the noun normally stays in its basic form.

In tiom da metalo, the noun metalo is part of a quantity expression. The same is true for vitro in kiom da vitro. The whole phrase functions as the object, but the noun after da itself does not take -n.

So this is normal:

• Ni kolektis tiom da metalo...
• not Ni kolektis tiom da metalon...

This is one of the important things to remember about da phrases in Esperanto.

kolektis is past tense.

The ending -is always marks the past tense in Esperanto.

So:

• kolektas = collect / are collecting
• kolektis = collected
• kolektos = will collect

Here Ni kolektis means We collected.

No. Here kiom is not asking a question.

Esperanto words like kiom can appear both in questions and in comparisons.

For example:

• Kiom da akvo vi trinkis? = How much water did you drink?
  Here it is a question.

• Mi trinkis tiom da akvo kiom vi. = I drank as much water as you.
  Here it introduces the second half of a comparison.

So in your sentence, kiom means as much as, not how much?

Yes, you may sometimes see a comma there, but it is often omitted in a short sentence like this.

So both of these are possible:

• Ni kolektis tiom da metalo kiom da vitro.
• Ni kolektis tiom da metalo, kiom da vitro.

The version without the comma is very natural, especially when the comparison is short and closely connected.

Because this sentence compares quantity, not manner or quality.

Use:

• tiom ... kiom ... for amount or number
• tiel ... kiel ... for the way something is done, or for degree in a different sense

Examples:

• Mi havas tiom da mono kiom vi. = I have as much money as you.
• Li kantas tiel bone kiel ŝi. = He sings as well as she does.

Since metal and glass are being compared by amount, tiom ... kiom ... is the right pattern.

Here it means the material glass.

That is an important vocabulary point:

• vitro = glass as a material
• glaso = a glass, drinking glass, tumbler

So:

• da vitro = of glass
• du glasoj = two glasses

In your sentence, vitro clearly means the substance collected, not cups or drinking glasses.

Yes, Esperanto word order is fairly flexible, though the original version is the most straightforward.

For example, you could also say:

• Tiom da metalo kiom da vitro ni kolektis.

That still means the same thing, but it sounds more marked because the quantity comparison is moved to the front.

The basic order:

• Ni = subject
• kolektis = verb
• tiom da metalo kiom da vitro = object/comparison phrase

is the clearest and most neutral.

No, not here.

After a quantity word like tiom, Esperanto normally uses da, not de, when talking about an amount of something.

So you say:

• tiom da metalo
• multe da akvo
• iom da tempo

not:

• tiom de metalo

Very roughly:

• da is used for quantity expressions
• de has many other meanings, such as of, from, by

So in this sentence, da is the correct choice.