En mia sako estas botelo da akvo, du pomoj kaj unu banano.

This is a very natural Esperanto way to present location first and then say what is there:

En mia sako estas ... = In my bag there is/are ...

It is an existential style, similar to English There is/There are .... The speaker is setting the scene first: in my bag.

You could also rearrange the sentence, but this version is especially natural when the location is the main starting point.

Esperanto does have havi = to have, so you could say:

Mi havas en mia sako botelon da akvo, du pomojn kaj unu bananon.

That would mean something like I have in my bag a bottle of water, two apples, and one banana.

But En mia sako estas ... is not the same structure. It means In my bag there is/are ... and focuses on what is located there, not on possession. Both are possible, but this sentence is using the there is/are style.

Because Esperanto verbs do not change according to number or person.

So:

• mi estas = I am
• ili estas = they are
• unu pomo estas = one apple is
• du pomoj estas = two apples are

The verb always stays estas in the present tense. This is much simpler than English, where is and are are different.

Because mia sako is part of the prepositional phrase en mia sako.

After a preposition like en, you normally do not use the accusative -n. So:

• en mia sako = in my bag
• sur la tablo = on the table
• sub la seĝo = under the chair

The accusative -n is mainly for direct objects, and sometimes for motion toward a place.

You use en mian sakon when there is movement into the bag.

Compare:

• En mia sako estas botelo. = The bottle is in my bag.
• Mi metas botelon en mian sakon. = I put a bottle into my bag.

So:

• en mia sako = location
• en mian sakon = direction/motion into

This is a very important Esperanto pattern.

Da is used for quantity or measure, often like English of in phrases such as:

• glaso da akvo = glass of water
• botelo da akvo = bottle of water
• kilo da pomoj = kilo of apples

So botelo da akvo means a bottle whose contents are water.

This is different from de, which often shows possession, origin, or relation. In this sentence, da is the correct choice because the bottle contains water.

Here akvo is a mass noun, like water in English.

It does not need plural -j, because we are not counting separate waters. We are just talking about the substance.

It also does not take accusative -n here, because it is inside the phrase botelo da akvo. In this expression, akvo is not a direct object.

So:

• botelo da akvo = bottle of water
• not botelo da akvon

Because du pomoj is plural, while unu banano is singular.

Esperanto marks plural with -j:

• pomo = apple
• pomoj = apples
• banano = banana
• bananoj = bananas

Since there are two apples, you need pomoj. Since there is one banana, you use singular banano.

Yes, it could be just banano in some contexts, but unu banano specifically says one banana.

Using unu makes the number explicit and matches the pattern of the list:

• botelo da akvo
• du pomoj
• unu banano

So it sounds neat and clear. Without unu, the sentence would still be grammatical, but slightly less specific in counting style.

Because Esperanto has no indefinite article.

English says:

• a bottle
• a banana

Esperanto simply says:

• botelo
• banano

Esperanto only has the definite article la, which means the.

So:

• botelo = a bottle / bottle
• la botelo = the bottle

In this sentence, there is no la because the items are being introduced, not identified as specific known objects.

Because they are not direct objects here.

In En mia sako estas botelo da akvo, du pomoj kaj unu banano, the sentence is saying what exists in the bag. The listed items are the things that are there, not things being acted on by another verb.

Compare:

• En mia sako estas du pomoj. = There are two apples in my bag.
• Mi manĝas du pomojn. = I eat two apples.

In the second sentence, pomojn gets -n because it is the direct object of manĝas. In your sentence, no accusative is needed.

No, that would be incorrect.

The forms pomojn and bananon are accusative, which would make them direct objects. But in this sentence, the apples and banana are not direct objects. They are simply the things that are in the bag.

So the correct forms are:

• du pomoj
• unu banano

not:

• du pomojn
• unu bananon