Sur la breto kuŝas ankaŭ alia sako, kaj mia ŝlosilo estas apud ĝi.

Esperanto word order is fairly flexible. Starting with Sur la breto puts the location first, a bit like On the shelf lies... in English.

So:

• Sur la breto kuŝas ankaŭ alia sako = On the shelf there is also another bag / Also another bag is lying on the shelf
• Ankaŭ alia sako kuŝas sur la breto would also be correct

Putting the place first often makes the sentence sound more natural or more descriptive.

Yes. In Esperanto, kuŝi means to lie or to be lying/resting in a horizontal position, and it can be used for objects as well as people or animals.

So a bag can kuŝi on a shelf if it is physically lying there.

Compare:

• La sako estas sur la breto = The bag is on the shelf
• La sako kuŝas sur la breto = The bag is lying on the shelf

Using kuŝas gives a more visual, physical sense of the bag’s position.

Not necessarily. They do different jobs:

• alia = another / other
• ankaŭ = also / too

So ankaŭ alia sako means something like also another bag or another bag as well.

This makes sense if the context already mentioned something else on the shelf, and now the speaker is adding that there is one more bag there too.

This is an important distinction:

• alia sako = another bag / a different bag
• la alia sako = the other bag

In this sentence, alia sako suggests an additional bag, not a specific one already contrasted with another known bag.

Because both phrases describe location, not motion toward somewhere.

In Esperanto:

• sur la breto = on the shelf
• apud ĝi = next to it

No accusative -n is used because nothing is moving.

You would use -n with these prepositions if there were movement toward a place:

• Mi metas ĝin sur la breton = I put it onto the shelf

So:

• sur la breto = on the shelf
• sur la breton = onto the shelf

Ĝi refers to alia sako — the bag.

So:

• mia ŝlosilo estas apud ĝi = my key is next to it
• it = the bag

In Esperanto, ĝi is the normal pronoun for things, objects, and sometimes animals when sex is not relevant.

Yes, it is completely normal. For objects, ĝi is exactly what Esperanto uses.

English speakers sometimes hesitate because English it can feel a bit different depending on context, but in Esperanto this is straightforward:

• person: li, ŝi, sometimes ri
• thing/object: ĝi

So using ĝi for sako is standard and natural.

Because possessive words like mia, via, lia, ŝia, and so on usually make the noun definite by themselves.

So Esperanto normally says:

• mia ŝlosilo = my key

not:

• la mia ŝlosilo

Using la here would usually sound unnatural. In ordinary Esperanto, the possessive already does the job that the often does in English.

Because estas apud ĝi is a neutral way to say where the key is.

• mia ŝlosilo estas apud ĝi = my key is next to it

If you said:

• mia ŝlosilo kuŝas apud ĝi

that would also be correct, but it would emphasize that the key is physically lying there.

So the difference is mostly one of style and nuance:

• estas apud ĝi = just location
• kuŝas apud ĝi = location plus a more visual sense of position

Not always. Esperanto punctuation is somewhat flexible here.

The comma in:

• Sur la breto kuŝas ankaŭ alia sako, kaj mia ŝlosilo estas apud ĝi.

is acceptable because it separates two full clauses. But many writers would also leave it out:

• Sur la breto kuŝas ankaŭ alia sako kaj mia ŝlosilo estas apud ĝi.

The version with the comma is often clearer and easier to read.

Yes, in effect it is.

English often uses there is / there are, but Esperanto often just states the location and then the verb:

• Sur la breto kuŝas alia sako
  literally: On the shelf lies another bag naturally: There is another bag on the shelf

So Esperanto does not always need a separate dummy subject like English there. It often simply places the location first and then the verb and subject.