Ĉi tiu merkato estas granda, kaj ĉi tie oni trovas freŝajn pomojn.

Tiu means that.
Ĉi tiu means this.

Esperanto often builds this with tiu plus the particle ĉi:

• tiu = that one
• ĉi tiu = this one

So:

• ĉi tiu merkato = this market
• tiu merkato = that market

You may also see tiu ĉi, which means the same thing, though ĉi tiu is very common.

They both contain ĉi, which adds the idea of nearness: this / here rather than that / there.

But they are built from different words:

• ĉi tiu = this
• ĉi tie = here

So:

• tiu = that
• ĉi tiu = this
• tie = there
• ĉi tie = here

In the sentence:

• Ĉi tiu merkato = this market
• ĉi tie = here

Esperanto does have a definite article: la = the.

But unlike English, Esperanto does not always need it in the same places. In this sentence, ĉi tiu already makes the noun specific:

• ĉi tiu merkato = this market

Because this already identifies the market, la is unnecessary. You would not normally say la ĉi tiu merkato.

Because granda describes the subject of the sentence, not a direct object.

In Ĉi tiu merkato estas granda:

• merkato is the subject
• estas means is
• granda is a predicate adjective describing the subject

Predicate adjectives after esti do not take -n.

Compare:

• La merkato estas granda. = The market is big.
• Mi vidas grandan merkaton. = I see a big market.

In the second sentence, grandan merkaton is the direct object, so it takes -n.

Because pomojn is the direct object of trovas.

In oni trovas freŝajn pomojn:

• oni = one / people / they in a general sense
• trovas = find
• pomojn = apples

The thing being found is the direct object, so Esperanto adds -n:

• pomo = apple
• pomoj = apples
• pomojn = apples as a direct object

This -n ending is called the accusative.

Adjectives in Esperanto must agree with the nouns they describe.

Since pomojn is:

• plural: -j
• accusative: -n

the adjective must also be:

• plural: -j
• accusative: -n

So:

• freŝa pomo = a fresh apple
• freŝaj pomoj = fresh apples
• freŝajn pomojn = fresh apples as a direct object

This agreement is very regular in Esperanto.

Oni is an indefinite subject. It often means:

• one
• people
• they
• someone
• you in a general sense

So oni trovas freŝajn pomojn means something like:

• one finds fresh apples
• people find fresh apples
• you can find fresh apples

It is used when the speaker does not want to name a specific person.

Because Esperanto uses the present ending -as for present-time statements and general truths.

• trovas = finds / find
• trovis = found
• trovos = will find

Here, the sentence is making a general present statement about the market:

• ĉi tie oni trovas freŝajn pomojn = here one finds fresh apples / here you can find fresh apples

English may use can find, but Esperanto often simply uses the present tense without a separate modal verb in this kind of statement.

Yes. Esperanto word order is fairly flexible, because endings show the grammatical roles.

This sentence uses a very natural order:

• ĉi tie = here
• oni = people / one
• trovas = find
• freŝajn pomojn = fresh apples

But other orders are possible, especially for emphasis, for example:

• Oni trovas freŝajn pomojn ĉi tie.
• Freŝajn pomojn oni trovas ĉi tie.

The original order sounds neutral and natural.

Kaj means and.

It joins the two parts of the sentence:

• Ĉi tiu merkato estas granda
• kaj ĉi tie oni trovas freŝajn pomojn

So the full sentence links two ideas:

1. the market is big
2. here people find fresh apples

These letters have regular sounds:

• ĉ sounds like ch in church
• ŝ sounds like sh in shoe

So:

• ĉi sounds roughly like chee
• freŝa sounds roughly like FRESH-ah

Also:

• j often sounds like English y
• ajn in freŝajn is pronounced as part of the ending, roughly eye-n or ahyn, depending on accent and speed

A rough pronunciation of freŝajn pomojn is:

• FRESH-ain po-MOYN

Yes. Merkato is a clear cognate for English speakers.

• merkato = market

Many Esperanto words come from familiar European roots, so they are often easy to recognize. But Esperanto gives them regular endings:

• noun ending: -o
• adjective ending: -a
• plural: -j
• accusative: -n

So from merkato, you could also get:

• merkatoj = markets
• merkatan = market-related / market as an adjective in accusative singular, depending on use

Not with exactly the same structure or emphasis.

• ĉi tie oni trovas freŝajn pomojn means here people find fresh apples or here you can find fresh apples
• ĉi tie estas freŝaj pomoj means there are fresh apples here

Both are possible sentences, but they say slightly different things:

• oni trovas... focuses on what is available to find or buy
• estas... simply states that the apples are there

In a market context, oni trovas freŝajn pomojn sounds very natural.