La diagonal del rectángulo va de una esquina a otra.

Because del is the mandatory contraction of de + el.

• de + el = del
• So la diagonal del rectángulo means the diagonal of the rectangle

This contraction happens only with el:

• de el libro → del libro
• a el parque → al parque

But it does not happen with la:

• de la casa stays de la casa

Yes, va literally means goes, from the verb ir. But in Spanish, ir can also be used in a descriptive, spatial way, not only for movement.

Here, va de una esquina a otra means something like:

• goes from one corner to another
• runs from one corner to another
• extends from one corner to another

In geometry or descriptions of lines, this is very natural Spanish.

This is a very common Spanish pattern:

• de ... a ... = from ... to ...

So:

• de una esquina a otra = from one corner to another

You will see the same structure in many contexts:

• de Madrid a Valencia
• de principio a fin
• de una puerta a otra

It is the standard way to mark the starting point and ending point.

In this sentence, otra means another or the other one, and Spanish often leaves out the repeated noun when it is obvious.

So:

• de una esquina a otra literally means
• from one corner to another

The word esquina is understood after otra.

You could think of it as:

• de una esquina a otra esquina

But repeating esquina would sound less natural here.

As for otra vs la otra:

• otra is the most natural general wording here
• la otra would sound more specific, as if the two corners had already been clearly identified

Spanish uses definite articles very often, sometimes more often than English does.

Here La diagonal means The diagonal. Spanish prefers the article because it is talking about a specific diagonal belonging to the rectangle being discussed.

This is very normal in Spanish:

• La puerta está abierta = The door is open
• El coche es rojo = The car is red

In context, Spanish often sounds more natural with the article than with no article.

The accent mark shows where the stress goes: rec-TÁN-gu-lo.

Without the accent, Spanish spelling rules would make you stress it differently. The written accent tells you the stressed syllable is tán.

So:

• rectángulo → stress on tán

This is useful both for pronunciation and for correct spelling.

Because esquina means corner, while ángulo means angle.

In English, when describing a rectangle, we often talk about a line going from one corner to another. Spanish does the same here, so esquina is the natural choice.

Compare:

• esquina = corner
• ángulo = angle

A rectangle has four ángulos as geometric angles, but if you mean the physical corner points, esquina is the usual word.

Yes, you could, and the meaning is basically the same.

Compare:

• va de una esquina a otra
• va desde una esquina hasta otra

Both mean goes from one corner to another.

However, de ... a ... is shorter and more natural in a simple geometric description like this.
desde ... hasta ... is also correct, but slightly more emphatic or explicit.

Va is the third person singular present tense of ir.

The full verb forms are:

• yo voy
• tú vas
• él/ella/usted va

The subject here is La diagonal, which is singular, so the verb must be va.

So the structure is:

• La diagonal = subject
• va = verb
• de una esquina a otra = complement showing direction/extent

Yes, but the original sentence is very natural.

Standard order:

• La diagonal del rectángulo va de una esquina a otra.

You might also hear variations such as:

• La diagonal va de una esquina a otra del rectángulo.

But the original is clearer and more elegant, because it identifies the rectangle early and then describes the diagonal.

For a learner, the given sentence is an excellent model.

Yes. Spanish often uses de where English might use ’s or a noun modifier.

So:

• La diagonal del rectángulo = the diagonal of the rectangle
• in more natural English, often the rectangle’s diagonal

Spanish normally prefers the de structure here rather than something equivalent to an English possessive ending.