Si trazas una diagonal dentro del rectángulo, verás dos triángulos iguales.

Si means if. It introduces a condition:

• Si trazas una diagonal... = If you draw a diagonal...

This is a very common Spanish pattern for real or possible situations.

In Spanish, when si means if, it is normally followed by the indicative, not the subjunctive, in sentences like this one.

Trazas is the tú form of trazar in the present indicative:

• yo trazo
• tú trazas
• él/ella traza

Even though English often says If you draw..., Spanish also uses the present tense here for a real condition. This is completely normal.

So:

• Si trazas una diagonal... = If you draw a diagonal...

It does not mean only you are drawing. In this kind of sentence, the present tense expresses a general or real condition.

Yes, verás is the future tense of ver:

• verás = you will see

Spanish often uses this future after a condition to show the result:

• Si trazas una diagonal, verás... = If you draw a diagonal, you will see...

In many situations, ves could also sound natural:

• Si trazas una diagonal, ves dos triángulos iguales.

That version feels more like a general truth or immediate result.
Using verás makes the consequence sound slightly more explicit: do this, and you will see.

Both trazas and verás are second person singular informal: tú.

Spanish usually leaves out subject pronouns when they are clear from the verb ending.

So the sentence really means:

• Si (tú) trazas una diagonal..., (tú) verás...

You could include tú, but it is usually unnecessary unless you want emphasis.

Trazar means something like:

• to trace
• to draw
• to draw out
• to mark

In geometry or technical descriptions, trazar is very common because it sounds precise: you are drawing a line or shape in a controlled way.

So:

• trazar una diagonal = to draw/trace a diagonal

Dibujar is more general and often means to draw in an artistic or broad sense.
In a geometry sentence, trazar is often the more natural choice.

This is about articles and contraction.

• una diagonal = a diagonal
  Here, it is introducing one diagonal, not a specific one already mentioned.

• del rectángulo = of the rectangle / inside the rectangle

Del is the contraction of de + el:

• de + el = del

So:

• dentro del rectángulo = inside the rectangle

You cannot say de el rectángulo here; in normal Spanish, it must become del rectángulo.

Dentro de means inside.

So:

• dentro del rectángulo = inside the rectangle

This phrase makes it clear that the diagonal is being drawn within the shape.

You might also hear:

• en el rectángulo = in/on the rectangle

But dentro del rectángulo is more specific and emphasizes the line is inside the boundaries of the rectangle.

Because diagonal here is a noun in Spanish, and it is feminine:

• la diagonal
• una diagonal

Many Spanish nouns ending in -al are masculine, but not all. Gender has to be learned word by word.

So in this sentence:

• una diagonal is correct
• not un diagonal

Because iguales is an adjective describing triángulos, and adjectives must agree in number (and often gender) with the noun.

• singular: un triángulo igual would only work in a different context
• plural: dos triángulos iguales

Here, triángulos is masculine plural, and iguales is the plural form that matches it.

Also notice that igual has the same form for masculine and feminine in the singular:

• un triángulo igual
• una figura igual

But in the plural it becomes:

• iguales

Here it means equal in the geometric sense: the two triangles are the same in size and shape.

Spanish igual / iguales can mean either:

• equal
• same
• identical, depending on context

In a maths or geometry sentence like this, dos triángulos iguales is naturally understood as two equal triangles.

Because Spanish, like English, often does not use a definite article when introducing something as a result:

• verás dos triángulos iguales = you will see two equal triangles

It would sound strange here to say verás los dos triángulos iguales unless you were referring to two specific triangles already known in the conversation.

So the sentence is simply presenting what appears when you draw the diagonal.

The accents show which syllable is stressed.

• rectángulo → stress on tán
• triángulos → stress on án
• verás → stress on the last syllable, rás

These accents are important for correct pronunciation and are required by Spanish spelling rules.

A rough stress guide:

• rec-TÁN-gu-lo
• tri-ÁN-gu-los
• ve-RÁS

Yes. It is a real or possible condition.

Structure:

• Si + present indicative, future
  Si trazas..., verás...

This is used for something realistic and straightforward: if the condition is met, the result will happen.

Very common patterns are:

• Si + present, future
• Si + present, present

Examples:

• Si estudias, aprobarás.
• Si estudias, apruebas.

The sentence you have follows that same pattern.

Yes. Spanish can change the order of the two clauses.

You can say:

• Si trazas una diagonal dentro del rectángulo, verás dos triángulos iguales.
• Verás dos triángulos iguales si trazas una diagonal dentro del rectángulo.

Both are correct.
The original version is probably preferred because it presents the condition first and then the result, which is very clear for an instruction or explanation.