El albañil dijo que la grieta en la pared se puede arreglar hoy mismo.

Not exactly.

In Spain, albañil usually means a bricklayer, mason, or more generally a building worker who does practical construction work such as walls, plaster, cement, tiles, and repairs.

Constructor is not the normal everyday word for this person. It usually relates more to a builder, developer, or even a construction company.

So in this sentence, el albañil is the workman who looked at the wall and gave his opinion.

El suggests a specific builder or workman that the speaker and listener already know about, or that has already been mentioned.

• El albañil = the builder / the workman
• Un albañil = a builder / a workman

So this sentence sounds like it is talking about a particular person, not just any builder.

Dijo is the preterite of decir and means the person said it as a completed event in the past.

It is used because the sentence reports one finished act of speaking.

Compare:

• dijo = he said
• decía = he was saying / he used to say
• ha dicho = he has said

In this sentence, dijo is the most neutral way to report what he said.

After verbs like decir, Spanish normally uses que to introduce what someone said.

So:

• dijo que... = he said that...

In English, that is often dropped, but in Spanish que is usually kept.

Also, with decir, you do not use de here. So dijo que is correct, while dijo de que would be wrong in this sentence.

En la pared focuses on the location of the crack: the crack is in / on the wall.

That is the most natural choice here, because we are talking about a physical defect located in the wall.

De la pared can sometimes be possible in other contexts, but it sounds more like the wall’s crack, which is less natural here.

So for a real, visible crack, la grieta en la pared is a very normal phrase.

This is a very common Spanish structure.

La grieta ... se puede arreglar means the crack can be fixed.

Here:

• puede = can
• arreglar = fix
• se helps create a very natural passive-like meaning in Spanish

Spanish often prefers this kind of structure instead of a more formal passive.

So rather than something like puede ser arreglada, Spanish more naturally says se puede arreglar.

The subject is la grieta.

That is the thing being talked about, and the verb agrees with it in the singular:

• la grieta ... se puede arreglar

Even though se can make the sentence feel less direct, se is not the subject.
It is just part of the construction that gives the meaning can be fixed.

Yes. Se puede arreglar and puede arreglarse both mean the same thing here.

With a conjugated verb plus an infinitive, Spanish often allows the pronoun to go in two places:

• se puede arreglar
• puede arreglarse

Both are correct.
Se puede arreglar is often a little more common in everyday speech, but both sound natural.

Both can work, but arreglar is more everyday and flexible.

• arreglar = fix, sort out, put right
• reparar = repair

For a crack in a wall, both are possible, but arreglar sounds very natural in normal conversation.

So se puede arreglar is a very idiomatic way to say it can be fixed.

Hoy mismo adds emphasis.

It means something like:

• today itself
• today, no later
• as soon as today

So it is stronger than just hoy.

Compare:

• hoy = today
• hoy mismo = today, and in fact right away / without waiting

It suggests there is no need to postpone the repair.

Spanish uses definite articles very naturally when talking about specific things, and often a bit more often than English does.

In this sentence:

• el albañil = a specific workman
• la grieta = the specific crack being discussed
• la pared = the wall where that crack is

So the articles make the sentence sound concrete and grounded in a real situation.