Der Abend war wunderbar.

Der is the definite article meaning the.

In this sentence, Abend is a masculine noun, and it is the subject of the sentence, so German uses der:

• der Abend = the evening

If the noun were feminine or neuter, the article would be different:

• die Nacht = the night
• das Wetter = the weather

Abend is masculine, so its dictionary form is der Abend.

Yes, in many cases you do need to learn German nouns together with their articles:

• der Abend = the evening
• der Tag = the day
• die Zeit = the time
• das Jahr = the year

Learning the article together with the noun is very important, because gender affects:

• articles
• adjective endings
• pronouns
• some case forms

So it is best to memorize der Abend, not just Abend.

In German, all nouns are capitalized, not just proper names.

So Abend gets a capital A because it is a noun.

Compare:

• der Abend = the evening
• am Abend = in the evening

This is a general rule in German and one of the most noticeable differences from English.

War is the simple past form of the verb sein (to be).

So:

• sein = to be
• ich war = I was
• er war = he was
• der Abend war = the evening was

In this sentence, war is used because the sentence is talking about the evening as something completed in the past.

Because the sentence is describing the evening as a past event.

• Der Abend war wunderbar. = The evening was wonderful.
• Der Abend ist wunderbar. = The evening is wonderful.

The version with ist would sound like the evening is happening now or is being described in the present.
The version with war is the natural choice when you are talking about an evening that has already happened.

Because German often places the conjugated verb in the second position in a normal main clause, and other information comes after it.

The structure here is:

• Der Abend = subject
• war = verb
• wunderbar = predicate adjective

So the basic word order is:

Subject + verb + complement

This is very similar to English:

• The evening was wonderful.

Because here wunderbar is not directly in front of a noun. It is being used as a predicate adjective, after the verb sein.

In German, predicate adjectives usually do not take endings:

• Der Abend war wunderbar.
• Das Essen ist gut.
• Die Blumen sind schön.

But if the adjective comes before a noun, it usually does take an ending:

• ein wunderbarer Abend = a wonderful evening
• der wunderbare Abend = the wonderful evening

So:

• Der Abend war wunderbar. → no adjective ending
• ein wunderbarer Abend → adjective ending required

Yes, wunderbar is a common and very natural word. It often means:

• wonderful
• marvelous
• lovely
• great

Depending on context, it can sound warm, positive, and sometimes a little expressive.

Examples:

• Das war wunderbar. = That was wonderful.
• Ein wunderbarer Abend. = A wonderful evening.
• Wunderbar! = Wonderful! / Great!

Yes, but that is not a full sentence by itself in the same way.

• Der Abend war wunderbar. = a complete sentence
• Ein wunderbarer Abend. = a noun phrase, meaning a wonderful evening

You could use Ein wunderbarer Abend as:

• a title
• a short exclamation
• part of a longer sentence

For example:

• Es war ein wunderbarer Abend. = It was a wonderful evening.

A simple learner-friendly guide would be:

• Der ≈ dare (but with a lighter r)
• Abend ≈ AH-bent
• war ≈ vahr
• wunderbar ≈ VOON-der-bahr

A few helpful notes:

• German w sounds like English v
• German vowel sounds are usually clearer and more consistent than in English
• the r may sound softer or more back in the throat, depending on accent

So the whole sentence sounds roughly like:

dare AH-bent vahr VOON-der-bahr

You can absolutely use it in everyday conversation. It sounds natural and normal.

For example:

• Der Abend war wunderbar. Danke noch mal! = The evening was wonderful. Thanks again!
• Gestern war wirklich schön. Der Abend war wunderbar. = Yesterday was really nice. The evening was wonderful.

It is neither especially formal nor especially casual. It works well in standard spoken and written German.

German often expresses this idea differently from English.

English commonly says:

• It was a wonderful evening.

German often prefers:

• Es war ein wunderbarer Abend.
• Der Abend war wunderbar.

So your sentence makes the evening the subject directly, instead of using a dummy subject like it.

That is completely normal in German.

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

You can tell this from both the function and the article:

• der Abend = nominative masculine

If Abend were used in a different role, the article might change:

• Ich sehe den Abend. = I see the evening. → accusative
• mit dem Abend = with the evening → dative

But in Der Abend war wunderbar, der Abend is the thing being described, so it is nominative.