O apartamento de que te falei fica perto do mercado, mas longe da escola.

Because the verb falar here is used with the preposition de: falar de alguém / de alguma coisa = to talk about someone / something.

So when that noun becomes part of a relative clause, the preposition must stay:

• Falei-te do apartamento. = I told you about the apartment.
• O apartamento de que te falei... = The apartment that I told you about...

In English, the preposition often moves to the end: the apartment I told you about.
In Portuguese, it normally stays before que: de que.

So:

• correct: o apartamento de que te falei
• not standard: o apartamento que te falei

Here de que means something like about which or that ... about.

A very literal version would be:

• O apartamento de que te falei
  = The apartment about which I told you

That sounds formal in English, but it helps show the grammar.

So de comes from the verb pattern falar de, and que is the relative word linking apartamento to the clause te falei.

In European Portuguese, object pronouns often come after the verb in affirmative main clauses:

• Falei-te ontem. = I spoke to you yesterday.

But some words trigger the pronoun to come before the verb. One of those triggers is a relative word like que.

So in this sentence:

• de que te falei

the que causes proclisis (pronoun before the verb), so te falei is the expected form.

Compare:

• Falei-te do apartamento.
• O apartamento de que te falei...

Both are correct; the position changes because of the grammar around the verb.

Te means you as an indirect/direct object pronoun in the informal singular form.

Here it means to you:

• falei-te / te falei = I told you / I spoke to you

So:

• de que te falei = that I told you about

If speaking formally to one person, Portuguese would usually use another structure, for example:

• de que lhe falei = that I told you about (formal)

Falei is the pretérito perfeito of falar in the eu form.

• eu falei = I spoke / I told

It refers to a completed past action: the speaker already told the listener about the apartment.

So:

• de que te falei = that I told you about

This is very common when referring back to something already mentioned.

Fica often means is located or lies when talking about location.

So:

• O apartamento fica perto do mercado
  = The apartment is located near the market

Using ficar for location is extremely common in Portuguese.

If you said está perto do mercado, it would also be understandable, but fica is often more natural when describing where a place is situated in a general sense.

So a useful distinction is:

• ficar = location, where something is situated
• estar = state or temporary position, though it can also describe location in some contexts

In this sentence, fica sounds very natural.

Because perto and longe are used with the preposition de:

• perto de
• longe de

Then de combines with the definite article:

• de + o = do
• de + a = da

So:

• perto do mercado = near the market
• longe da escola = far from the school

This contraction is required.

Portuguese often uses definite articles with nouns where English may or may not use them.

Here we have:

• o mercado
• a escola

After de, they become:

• do mercado
• da escola

So the full patterns are:

• perto de o mercado → perto do mercado
• longe de a escola → longe da escola

Using the article here is normal and natural in Portuguese.

In this sentence, perto and longe behave like location expressions, typically followed by de:

• perto de
• longe de

For a learner, the most useful thing is to remember them as fixed patterns rather than worry too much about labeling them.

Learn them like this:

• ficar perto de = to be near
• ficar longe de = to be far from

That will help you produce correct Portuguese more easily.

Yes, but it is more formal and less natural in everyday speech.

• O apartamento de que te falei... = normal, natural
• O apartamento sobre o qual te falei... = more formal, more written

Also, falar de is the most direct match here, so de que is the usual choice.

For most everyday situations, use:

• de que te falei

It breaks down like this:

• O apartamento = the apartment
• de que te falei = that I told you about
• fica = is located
• perto do mercado = near the market
• mas = but
• longe da escola = far from the school

So the structure is:

noun + relative clause + main verb + location phrase + contrast + location phrase

That is:

• O apartamento
• de que te falei
• fica perto do mercado, mas longe da escola.

Yes. Portuguese allows some variation, though the original is very natural.

For example:

• O apartamento de que te falei fica perto do mercado, mas fica longe da escola.

This repeats fica, which is grammatically fine, but the original version sounds smoother because Portuguese often omits repeated material when it is clear:

• fica perto do mercado, mas longe da escola

You could also hear:

• O apartamento de que te falei é perto do mercado?

But that sounds less natural in standard EP for this meaning. Fica is better when describing where a place is located.

Very close to exactly what you have.

A very natural everyday version is:

• O apartamento de que te falei fica perto do mercado, mas longe da escola.

In conversation, speakers might reduce sounds a bit in pronunciation, but grammatically this is already a normal and natural European Portuguese sentence.

The main things that make it especially good for EP are:

• de que after falar de
• te falei because que triggers pronoun-before-verb order
• fica for location
• do / da contractions after perto de / longe de