Wir beschreiben das Ereignis.

Because the subject is wir (we). In the present tense, beschreiben is conjugated like this:

• ich beschreibe
• du beschreibst
• er/sie/es beschreibt
• wir beschreiben
• ihr beschreibt
• sie/Sie beschreiben

With wir, the verb form is the same as the infinitive for many German verbs.

Ereignis is a neuter noun in German, so its singular definite article is das.

So:

• das Ereignis = the event

You have to learn the noun together with its article: das Ereignis, not just Ereignis.

It is in the accusative case, because it is the direct object of beschreiben.

In this sentence:

• Wir = subject
• beschreiben = verb
• das Ereignis = direct object

A useful detail: for many neuter nouns, the nominative and accusative forms look the same, so das Ereignis could be either form depending on the sentence.

This is the normal word order for a basic German main clause:

subject + finite verb + object

So:

• Wir = subject
• beschreiben = finite verb
• das Ereignis = object

German main clauses usually follow the verb-second rule, meaning the conjugated verb comes in the second position.

Yes. That is also correct German.

German lets you move elements to the first position for emphasis, as long as the finite verb still stays in second position:

• Wir beschreiben das Ereignis.
• Das Ereignis beschreiben wir.

The second version puts more focus on das Ereignis.

No. beschreiben is an inseparable verb.

It has the prefix be-, and be- is normally inseparable. So you do not split it up:

• correct: Wir beschreiben das Ereignis.
• not correct: Wir schreiben das Ereignis be.

A useful related point: inseparable verbs usually do not add ge- in the past participle, so:

• beschreiben → beschrieben

Because all German nouns are capitalized.

So:

• wir is not capitalized unless it starts the sentence
• Ereignis is capitalized because it is a noun

This is one of the most noticeable spelling differences between German and English.

The plural is die Ereignisse.

So:

• singular: das Ereignis
• plural: die Ereignisse

Example:

• Wir beschreiben die Ereignisse. = We describe the events.

In many contexts, German present tense can cover both ideas.

So Wir beschreiben das Ereignis can mean:

• We describe the event
• We are describing the event

German does not have a separate verb form exactly like the English progressive are describing. Usually the context tells you which meaning is intended.

Yes. Many nouns ending in -nis are neuter, so das Ereignis fits a common pattern.

Other examples:

• das Ergebnis = the result
• das Zeugnis = the certificate / report
• das Verhältnis = the relationship / ratio

This pattern is helpful, but you should still learn each noun with its article, because memorizing gender noun by noun is the safest approach.