La profesora explicó que un rectángulo tiene cuatro ángulos rectos y que un cuadrado también.

Because profesora is the feminine form of profesor.

• el profesor = a male teacher / the male teacher
• la profesora = a female teacher / the female teacher

In this sentence, the teacher is female, so Spanish uses la profesora.

Explicó is the preterite tense of explicar.

• explicar = to explain
• explicó = explained

The preterite is used here because it refers to a completed action in the past: the teacher explained something.

The accent mark in explicó shows the correct stress: ex-pli-CÓ.

It also helps distinguish forms in writing. For example:

• explico = I explain
• explicó = he/she explained

So the accent is important both for pronunciation and meaning.

Spanish often repeats que when joining two clauses after a verb like explicar, decir, or pensar.

So:

• explicó que un rectángulo tiene... y que un cuadrado también

means:

• she explained that a rectangle has... and that a square does too

The second que makes the structure clearer and sounds very natural.

Sometimes Spanish can omit a repeated que, but in this sentence keeping it is clearer and more natural.

• Natural: La profesora explicó que un rectángulo tiene... y que un cuadrado también.

If you remove the second que, the sentence can feel less neat or slightly harder to process. So for learners, it is best to treat the repeated que as the normal pattern here.

Because the subject is singular:

• un rectángulo = a rectangle

So the verb must also be singular:

• un rectángulo tiene = a rectangle has

If the subject were plural, then you would use tienen:

• los rectángulos tienen = rectangles have

Here, un rectángulo and un cuadrado are being used in a general, classifying way:

• un rectángulo tiene... = a rectangle has...
• un cuadrado también = a square does too

Spanish often uses the singular indefinite article this way when talking about a thing as a type or category. English does the same in sentences like A dog has four legs.

Because Spanish is leaving out repeated information that is already understood. This is called ellipsis.

The full version would be:

• La profesora explicó que un rectángulo tiene cuatro ángulos rectos y que un cuadrado también tiene cuatro ángulos rectos.

But repeating tiene cuatro ángulos rectos would sound unnecessary, so Spanish drops it.

In English we do the same:

• A rectangle has four right angles, and a square does too.

In this sentence, también means also / too, and putting it at the end is very natural because it refers to the whole omitted idea:

• un cuadrado también = a square does too

Spanish often places también before what it modifies, but with this kind of ellipsis, final position sounds very normal.

Compare:

• Un cuadrado también tiene cuatro ángulos rectos.
• Un cuadrado también.

Both are correct, but the second one is shorter because the rest is understood.

Literally, ángulos rectos means right angles.

• ángulo = angle
• recto = right / straight, depending on context

In geometry, ángulo recto specifically means a right angle.

So:

• cuatro ángulos rectos = four right angles

Because it describes ángulos, which is plural.

In Spanish, adjectives usually agree with the noun in gender and number:

• ángulo recto = one right angle
• ángulos rectos = right angles

So rectos is masculine plural to match ángulos.

They have accent marks because Spanish spelling shows where the stress falls.

• rectángulo → stress on tán
• ángulos → stress on án
• también → stress on bién

These accent marks are part of the correct spelling and pronunciation. In Spanish, they are not optional.

Because explicó que introduces a statement presented as information or fact, not as doubt, emotion, or uncertainty.

So Spanish uses the indicative:

• explicó que un rectángulo tiene...

The subjunctive is usually used after expressions of doubt, desire, emotion, or unreality, which is not the case here.

Yes. The definite article la suggests a specific teacher, not just any teacher.

• La profesora = the teacher (female)
• Una profesora = a teacher (female)

So this sentence means that a particular teacher explained it.