La mesa del salón parece un cuadrado desde arriba, aunque en realidad es casi un rectángulo.

Del is the contraction of de + el.

So:

• de el salón → del salón

This is mandatory in normal Spanish whenever de is followed by the masculine singular article el.

Here, la mesa del salón means the table of the living room, or more naturally in English, the living-room table / the table in the living room.

In Spain, salón commonly means living room or main sitting room.

So la mesa del salón is most naturally understood as the table in the living room.

A learner might know salón from words like salon, but in Spanish it does not usually mean a beauty salon in this context. In Spain, for a beauty salon you would normally say something more specific, such as salón de belleza or peluquería, depending on the type.

Because the sentence is talking about appearance, not reality.

• parece = it seems / it looks
• es = it is

So:

• La mesa del salón parece un cuadrado desde arriba = the table looks like a square from above
• aunque en realidad es casi un rectángulo = although in reality it is almost a rectangle

This contrast is important: it appears one way, but its real shape is slightly different.

This is a very common question.

Here, cuadrado and rectángulo are being used as nouns, not adjectives:

• un cuadrado = a square
• un rectángulo = a rectangle

The table is being compared to those shapes, so Spanish uses the masculine nouns for the shapes themselves.

It is not saying the table is square-shaped with an adjective. If it were using an adjective, you would expect feminine agreement:

• La mesa parece cuadrada = The table seems square-shaped

But in your sentence, the structure is:

• parece un cuadrado
• es casi un rectángulo

That means it seems to be a square / it is almost a rectangle, using shape nouns.

Yes, absolutely.

That version uses the adjective cuadrada to agree with mesa:

• La mesa parece cuadrada = The table looks square / square-shaped

Your original sentence, parece un cuadrado, is also correct, but it feels a bit more like comparing the table’s top view to the geometric figure itself.

So both are possible:

• parece cuadrada → uses an adjective
• parece un cuadrado → uses a noun phrase

The original wording is quite natural when talking about shapes in a visual or geometric way.

Desde arriba means from above or seen from above.

It tells you the point of view:

• if you look at the table from above, it seems square

Literally:

• desde = from
• arriba = above / up

In English, we often say from above or from the top. In Spanish, desde arriba is a very normal way to express that viewpoint.

Because here the speaker is presenting the second part as a real fact.

• aunque can be followed by the indicative when the information is presented as true or known
• it can be followed by the subjunctive when the information is hypothetical, uncertain, or not being asserted as a fact

In this sentence:

• aunque en realidad es casi un rectángulo

the speaker is saying this is the true situation, so es (indicative) is the natural choice.

If you said aunque sea..., it would suggest something less certain or more concessive in a different context.

En realidad means in reality, actually, or really.

Here it contrasts appearance with truth:

• parece un cuadrado = it looks like a square
• en realidad = but in fact / actually
• es casi un rectángulo = it is almost a rectangle

It is a very common phrase for correcting an impression or adding the true situation.

Because casi un rectángulo means almost a rectangle, which is slightly more precise and geometric.

Compare:

• es rectangular = it is rectangular
• es casi un rectángulo = it is almost a rectangle

The original wording suggests that the shape is very close to a true rectangle, perhaps not perfectly so, or that the speaker is thinking in terms of geometric figures.

Also, the sentence is balanced stylistically:

• parece un cuadrado
• es casi un rectángulo

Both parts use shape nouns, which makes the sentence feel neat and parallel.

Yes. Parecer can be followed by:

• an adjective: parece cuadrada
• a noun phrase: parece un cuadrado
• even a full clause in other contexts

So parece un cuadrado is a perfectly normal structure meaning it looks like a square or it seems to be a square.

Spanish often uses parecer + noun for comparisons of appearance:

• Eso parece un problema = That looks like a problem
• La nube parece un dragón = The cloud looks like a dragon

Because they do different jobs.

• la mesa refers to a specific table: the table in the living room
• un cuadrado and un rectángulo refer to types/categories of shape

So:

• la mesa = a known, specific object
• un cuadrado / un rectángulo = one example of that kind of figure

This is very similar to English:

• The table looks like a square
• although it is almost a rectangle

Yes. It sounds natural in Spain, especially because of salón for living room.

The whole sentence is standard Spanish, but salón is particularly typical in Spain for this room of the house. In other Spanish-speaking regions, people might more often say sala or sala de estar, depending on the country.

So for Spanish from Spain, the sentence fits very well.