Un mensaje bien entendido previene problemas.

Because entendido is the past participle of entender, and here it works like an adjective: understood.

So un mensaje bien entendido means a well-understood message.

By contrast, entendiendo is the gerund and means understanding. It would not fit here.

• entendido = understood
• entendiendo = understanding

Bien modifies entendido, so together they mean well understood.

This is very similar to English:

• well understood
• carefully prepared
• poorly written

So the structure is:

• mensaje = noun
• entendido = participle used adjectivally
• bien entendido = well understood

Because bien entendido is not the main verb of the sentence. It is part of the noun phrase un mensaje bien entendido.

The main verb is previene.

So the sentence breaks down like this:

• Un mensaje bien entendido = the subject
• previene = the verb
• problemas = the object

In English, this is like saying:

• A well-understood message prevents problems

Not:

• A message is well understood prevents problems

Yes. Since entendido is functioning like an adjective, it must agree in gender and number with mensaje.

Here:

• mensaje is masculine singular
• so we use entendido

Compare:

• un mensaje entendido
• una idea entendida
• mensajes entendidos
• ideas entendidas

Un is being used in a general, generic way: it means any message / a message in general.

Spanish often uses the singular indefinite article this way when making a general statement.

So:

• Un mensaje bien entendido previene problemas = a general truth

If you said El mensaje bien entendido, it would usually sound more specific, as if you were talking about a particular message.

Because the verb is prevenir, which is irregular and follows the pattern of venir.

Its present-tense forms include:

• yo prevengo
• tú previenes
• él/ella previene
• nosotros prevenimos
• vosotros prevenís
• ellos previenen

So previene is the correct third-person singular present form, agreeing with un mensaje.

Yes. It is the present indicative, third person singular.

In this sentence, the present tense expresses a general fact or general principle, not just something happening right now.

So it means something like:

• A well-understood message prevents problems
• A well-understood message tends to prevent problems

This is very common in both Spanish and English.

Yes, in many contexts evita problemas would be natural, but there is a slight nuance.

• prevenir problemas = to prevent problems, often by stopping them before they arise
• evitar problemas = to avoid problems

In this sentence, previene sounds a bit more formal and fits well with the idea of communication preventing issues before they happen.

Both are possible, but previene is a good choice here.

Possibly, yes, but entendido is the more natural and common choice here.

Both entender and comprender can relate to understanding, but:

• entender is more common in everyday Spanish
• comprender can sometimes sound slightly more formal or emphasize fuller comprehension

So:

• un mensaje bien entendido = very natural
• un mensaje bien comprendido = possible, but less common in this kind of general statement

Because bien normally goes before the adjective or participle it modifies in expressions like this.

So:

• bien entendido = well understood

That is the standard and natural order.

Putting bien after entendido would sound awkward here. Spanish generally prefers bien + participle/adjective in this kind of phrase.

Grammatically, it comes from a verb, but in this sentence it is functioning as an adjectival participle.

So it behaves more like an adjective than like a full verb.

That is why it:

• describes mensaje
• agrees with the noun
• appears without an auxiliary verb like haber

So in this sentence, it is best understood as an adjective formed from a participle.