Der Regenschirm ist heute notwendig.

Because Regenschirm is a masculine noun in German, so its nominative singular article is der.

A useful rule for compound nouns is that the last part determines the gender. Here, the last part is Schirm (umbrella/screen/shade), and Schirm is masculine. So the whole compound Regenschirm is also masculine.

It is in the nominative case because it is the subject of the sentence — the thing being talked about.

In Der Regenschirm ist heute notwendig, the subject is der Regenschirm, and the verb is ist.

German often makes compound nouns by joining smaller words together into one word.

Regen = rain
Schirm = umbrella / shield / screen

So Regenschirm literally means something like rain-umbrella, but in normal English we just say umbrella.

This is very common in German, and native English speakers often notice it quickly because English usually writes these as separate words or uses shorter established words.

Because German main clauses usually follow the verb-second rule, often called V2.

In this sentence:

Der Regenschirm = first element
ist = second element

That is exactly what German expects in a normal statement.

Even if you move another element to the front, the verb still stays in second position. For example:

Heute ist der Regenschirm notwendig.

Here heute is first, and ist is still second.

Because heute is a time adverb (today), and notwendig is a predicate adjective after sein.

A very normal German order is:

subject + sein + time expression + predicate adjective

So:

Der Regenschirm ist heute notwendig.

This sounds natural. German often places time expressions before the final descriptive part of the sentence.

You could also say:

Heute ist der Regenschirm notwendig.

That puts more emphasis on today.

Because here it is used after the verb sein and acts as a predicate adjective. Predicate adjectives in German do not take adjective endings.

So:

Der Regenschirm ist notwendig.

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

ein notwendiger Regenschirm

That difference is very important in German:

• after sein → no ending
• before a noun → adjective ending

Notwendig is correct, but in everyday speech many Germans would more often say nötig.

So these are both possible:

Der Regenschirm ist heute notwendig.
Der Regenschirm ist heute nötig.

Both mean basically the same thing here, but nötig often sounds a bit more common and conversational.

In very natural everyday German, people might also say something like:

Heute braucht man einen Regenschirm.
or
Ein Regenschirm ist heute nötig.

Because all nouns are capitalized in German, not just proper names.

So:

• der Regenschirm
• das Haus
• die Schule

This is one of the first spelling differences English speakers usually notice when learning German.

Heute is an adverb, specifically a time adverb. It means today.

It does not need an article, and it does not change form here. It simply tells you when something is true.

So in this sentence, heute answers the question when is the umbrella necessary?

Yes, but the meaning or emphasis may shift slightly.

For example, these are correct:

Der Regenschirm ist heute notwendig.
Normal statement.

Heute ist der Regenschirm notwendig.
Puts more focus on today.

What must stay true in a main clause is that the conjugated verb (ist) stays in second position.

So German word order is flexible, but not completely free.