Das Ereignis ist wichtig.

Because Ereignis is a neuter noun in German. The definite article for a neuter noun in the nominative singular is das.

So:

• der = masculine
• die = feminine
• das = neuter

In this sentence, das Ereignis is the subject, so it is in the nominative case.

It is in the nominative case, because it is the subject of the sentence.

The verb ist comes from sein and means is. With sein, the subject is in the nominative:

• Das Ereignis = subject
• ist wichtig = says something about the subject

There is no direct object in this sentence.

Because German main clauses normally follow the verb-second rule.

That means the conjugated verb goes in the second position:

• Das = first position
• ist = second position
• Ereignis wichtig = the rest

So Das Ereignis ist wichtig is standard German word order.

You can also move another element to the front, but the verb still stays second:

• Heute ist das Ereignis wichtig.

Because wichtig is being used as a predicate adjective, not an adjective directly before a noun.

After verbs like sein, werden, and bleiben, adjectives usually do not take endings:

• Das Ereignis ist wichtig.
• Die Situation ist schwierig.

Compare that with an adjective before a noun, where it does take an ending:

• das wichtige Ereignis

So:

• ist wichtig = no ending
• das wichtige Ereignis = adjective ending required

In this sentence, das is most naturally understood as the definite article: the.

German das can also be a demonstrative word, depending on context, and then it can feel like that or this in English. But here, with a noun right after it and no special emphasis, learners should read it as the normal article.

So in Das Ereignis ist wichtig, das is simply the article belonging to Ereignis.

Because all German nouns are capitalized.

So:

• das Ereignis
• der Tag
• die Frau

Also, Das is capitalized here because it is the first word of the sentence.

wichtig is not capitalized because it is an adjective, not a noun.

It is pronounced roughly like eh-RINE-nis, but with German sounds.

A more accurate guide:

• Er- sounds somewhat like air
• -eig- contains the diphthong ei, which sounds like English eye
• -nis sounds like niss

One thing learners often notice is that German speakers clearly separate the syllables, especially before ei.

The plural is die Ereignisse.

So:

• singular: das Ereignis
• plural: die Ereignisse

The full plural sentence would be:

• Die Ereignisse sind wichtig.

Notice that both the article and the verb change:

• das Ereignis ist
• die Ereignisse sind

Yes. That is also correct, but it is a different structure.

Compare:

• Das Ereignis ist wichtig.
  The noun is the subject, and wichtig is a predicate adjective.

• Das ist ein wichtiges Ereignis.
  Das is now standing on its own as that/this/it, and wichtiges comes before the noun, so it needs an adjective ending.

That is why you get:

• wichtig after ist
• wichtiges before Ereignis

Yes. It is completely normal, grammatical German.

It is a simple and neutral statement:

• subject: Das Ereignis
• verb: ist
• description: wichtig

It sounds straightforward and standard, which makes it a very useful model sentence for learners.