No entendemos el porqué del cambio.

Because in this sentence el porqué is a noun, meaning the reason, the cause, or literally something like the why.

So:

• No entendemos el porqué del cambio = We do not understand the reason for the change.

When Spanish uses porqué as a noun, it is normally written as one word and often appears with an article or another determiner:

• el porqué
• un porqué
• ese porqué

It has an accent because the noun porqué is stressed on the final syllable: por-QUÉ.

This helps distinguish it from other similar forms, especially porque, which usually means because.

So the accent is part of the spelling of the noun porqué.

This is one of the most common Spanish spelling questions.

• porqué = the reason, the motive
  It is a noun.
  Example: No entendemos el porqué del cambio.

• por qué = why / for what reason
  It is used in direct and indirect questions.
  Example: No entendemos por qué cambió todo.
  = We do not understand why everything changed.

• porque = because
  It gives an explanation.
  Example: Cambió porque era necesario.
  = It changed because it was necessary.

• por que = a less common combination of por

  • que
    It appears in certain structures, often more formal or literary.
    Example: Esa es la razón por que luchan.
    In modern everyday Spanish, learners will see this much less often.

So in your sentence, porqué is correct because it means the reason.

Because porqué is being used as a noun, and nouns in Spanish often take an article.

Here, el porqué works like:

• la razón
• el motivo
• la causa

So:

• el porqué del cambio
• la razón del cambio
• el motivo del cambio

They are all similar in structure.

Yes.

del = de + el

So:

• el porqué del cambio literally breaks down as
• the reason of the change

In natural English, that becomes the reason for the change.

Spanish uses de in many places where English prefers of or for.

Because Spanish usually leaves out subject pronouns when they are not needed.

The ending -emos already tells you the subject is we:

• entiendo = I understand
• entiendes = you understand
• entendemos = we understand

So No entendemos already means We do not understand.

You could say Nosotros no entendemos, but that would usually add emphasis or contrast, such as:

• Ellos sí lo entienden, pero nosotros no entendemos el porqué del cambio.
  They do understand it, but we do not understand the reason for the change.

Yes, comprender could also work here.

• No entendemos el porqué del cambio
• No comprendemos el porqué del cambio

Both mean something like We do not understand the reason for the change.

In many contexts, entender and comprender overlap a lot.
Very generally:

• entender is extremely common in everyday speech
• comprender can sometimes sound a bit more formal or more like fully grasp

But in this sentence, either is possible.

No.
If you want to say We do not understand each other, you need the reflexive/pronominal form:

• No nos entendemos

Your sentence is:

• No entendemos el porqué del cambio

Here entender simply means to understand something, not each other.

So the object is el porqué del cambio.

It is correct and natural, but it can sound slightly more formal, abstract, or written than other options.

A speaker might also say:

• No entendemos la razón del cambio
• No entendemos por qué hubo un cambio
• No entendemos por qué cambió

So el porqué is absolutely valid, but it has a somewhat more concise, noun-like, sometimes slightly more formal feel.

No. That would be incorrect.

If you use por qué as two words, it introduces a question clause, so it needs a full clause after it, for example:

• No entendemos por qué cambió todo
• No entendemos por qué hubo un cambio

But if you want a noun meaning the reason, then you need el porqué:

• No entendemos el porqué del cambio

So:

• por qué
  • clause
• el porqué
  • noun phrase

It is closer in structure to We don’t understand the reason for the change, because el porqué is a noun phrase.

But in meaning, it is very close to:

• We don’t know why it changed
• We don’t understand why the change happened

So the idea is the same, but the Spanish sentence is built around the reason, not around a full why clause.

Yes. As a noun, it can be pluralized:

• el porqué
• los porqués

For example:

• No conocemos los porqués de su decisión.
  We do not know the reasons behind his/her decision.

This is another sign that porqué here is a true noun.

Entendemos is the present indicative of entender.

Depending on context, it can mean:

• we understand
• we are understanding
  though in English this second option is much less common

In this sentence, the natural translation is simply:

• We do not understand the reason for the change

So it is a normal present-tense statement.