Ni metas la rubon en rubsakon.

It follows a very common Esperanto pattern:

• Ni = the subject, we
• metas = the verb, put / are putting
• la rubon = the direct object, the garbage
• en rubsakon = a prepositional phrase, into a garbage bag

So the basic structure is:

subject + verb + object + place/direction

That is very similar to normal English word order.

The -n marks the direct object in Esperanto.

Here, the thing being put is the garbage, so rubo becomes rubon.

• rubo = garbage
• rubon = garbage as the direct object

A useful question is: What are we putting?
Answer: the garbage.
So it gets -n.

This is a very common Esperanto feature.

Normally, a noun after a preposition does not take -n. But it can take -n to show direction toward a place.

So here:

• en rubsako would mean in a garbage bag as a location
• en rubsakon means into a garbage bag as movement/direction

Because the garbage is being moved from outside to inside the bag, rubsakon gets -n.

Compare:

• La rubo estas en la rubsako. = The garbage is in the garbage bag.
• Ni metas la rubon en la rubsakon. = We put the garbage into the garbage bag.

la means the.

So:

• la rubon = the garbage
• rubsakon = a garbage bag / some garbage bag

Esperanto has no separate word for a or an. Very often, if there is no la, the meaning is indefinite.

So this sentence suggests:

• the garbage is specific or already known
• the bag is not being identified as a specific known bag

If you wanted to say into the garbage bag, you could say:

Ni metas la rubon en la rubsakon.

Correct. Esperanto has a definite article, la, but no indefinite article.

That means:

• la rubo = the garbage
• rubo = garbage / some garbage
• la rubsako = the garbage bag
• rubsako = a garbage bag

English forces you to choose between the and a/an more often than Esperanto does.

Because en means in or into, while al means to or toward.

When you put something inside a bag, the idea is into, not just to.

So:

• en rubsakon = into a garbage bag
• al rubsako would sound more like movement toward the bag, not into it

With verbs of placing, Esperanto often uses a preposition that shows the final position:

• sur la tablon = onto the table
• en la skatolon = into the box
• sub la seĝon = under the chair

Yes, rubsako is naturally one word in Esperanto.

Esperanto forms compound words very freely. Here:

• rub- = garbage
• sako = bag

So rubsako means garbage-bag, or more naturally, garbage bag / trash bag.

In Esperanto, the main idea usually comes last. So rubsako is a kind of bag.

This is very normal in the language, and you usually do not need a hyphen.

Because -as is the present tense ending.

• metas = put / are putting
• metis = put / did put
• metos = will put

Esperanto present tense is broader than English. It can mean:

• we put
• we are putting
• sometimes even we do put

The exact English translation depends on context.

Yes. Enmeti means to put in / insert.

So Ni enmetas la rubon en rubsakon is possible and understandable.

But meti ... en ... is already perfectly normal:

• meti ion en ion = to put something into something

So the original sentence is simple and natural.

Yes, Esperanto word order is fairly flexible, because endings help show the grammar.

For example, you could say:

En rubsakon ni metas la rubon.

That still means basically the same thing, but now it emphasizes where the garbage is being put.

Even so, the most neutral and beginner-friendly order is the original one:

Ni metas la rubon en rubsakon.

Not quite.

• rubsako = a garbage bag, a bag for garbage
• ruba sako = literally garbage-related bag or trashy bag, which is less idiomatic and may suggest a bag characterized by ruba

In Esperanto, compounds like rubsako are usually the normal way to express this kind of idea. So rubsako is the better choice here.