El hecho es importante.

In Spanish, singular countable nouns usually need an article or some other determiner, just like in English you normally say the fact or a fact, not just fact.

So:

• el hecho = the fact
• un hecho = a fact

In this sentence, el tells you we are talking about a specific fact, not just the idea of factuality in general.

Without the article, Hecho es importante, it would sound wrong in normal Spanish.

Here, hecho is a noun meaning fact.

It is also related to the verb hacer (to do / to make). In fact, hecho is also the past participle of hacer:

• He hecho la comida = I have made the meal

But in El hecho es importante, it is not being used as a verb form. It is a standalone noun meaning fact, event, or thing that has happened / been established depending on context.

In this case, yes, hecho is masculine, and the article el shows that:

• el hecho importante
• los hechos importantes

Words ending in -o are very often masculine, although that is a pattern, not an absolute rule for every word in Spanish.

Here you should simply learn hecho as a masculine noun: el hecho.

Spanish usually uses ser for characteristics seen as essential, defining, or generally true, and estar for states or conditions.

Here, important is being presented as a quality of the fact itself, so ser is the natural choice:

• El hecho es importante. = The fact is important.

Using estar here would sound odd in most normal contexts. Importante is typically used with ser when you mean that something matters or has importance in a general sense.

Because importante is one of many Spanish adjectives that have the same form for masculine and feminine.

Compare:

• el hecho importante
• la decisión importante

The adjective changes only for number, not for masculine/feminine gender:

• el hecho importante
• los hechos importantes

So the final -e adjective form is normal here.

Because this sentence uses a copular structure:

• El hecho = subject
• es = verb
• importante = predicate adjective

So importante is not directly modifying the noun inside the noun phrase; it is describing the subject through the verb ser.

Compare:

• El hecho es importante. = The fact is important.
• El hecho importante... = The important fact...

Those are different structures.

Yes, it is grammatically possible, but it is less neutral and sounds more marked or emphatic.

• El hecho es importante. = the normal, straightforward order
• Es importante el hecho. = possible, but it gives a different focus, something like What is important is the fact

A learner should usually stick with El hecho es importante unless there is a special reason to change the word order.

In standard Spanish pronunciation:

• h is silent
• ch is like ch in church

So hecho sounds roughly like:

• EH-cho

More precisely: [ˈetʃo]

A native English speaker should be especially careful not to pronounce the h.

Yes. Hecho can have several related meanings depending on context, such as:

• fact
• event
• deed
• something done
• part of verb forms from hacer

For example:

• Es un hecho. = It is a fact.
• Los hechos ocurrieron ayer. = The events happened yesterday.
• Está hecho. = It’s done.

So in Spanish, hecho is a very common word with more than one use. In your sentence, though, the noun meaning fact is the one that matters.

The plural would be:

Los hechos son importantes.

The changes are:

• el → los
• hecho → hechos
• es → son
• importante → importantes

That shows agreement in article, noun, verb, and adjective for plural.

Yes, but it means something slightly different.

• El hecho es importante. = The fact is important
  This refers to a specific fact already known in the context.

• Un hecho es importante. = A fact is important / One fact is important
  This introduces a fact rather than referring to one already identified.

So both are grammatical, but el and un do not mean the same thing.