Die Idee ist prima.

Because Idee is a feminine noun in German. German nouns have grammatical gender, so you have to learn each noun together with its article:

• die Idee = the idea

In the nominative singular, the feminine definite article is die.

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

So:

• die Idee
• das Haus
• der Mann

This is one of the first big spelling differences English speakers notice.

This is normal German main-clause word order:

• Die Idee = subject
• ist = verb
• prima = complement

In a basic statement, the finite verb usually comes in second position in German. So the pattern here is:

• Subject + verb + description

That is why ist comes right after Die Idee.

Ist is the third-person singular form of the verb sein = to be.

So:

• ich bin = I am
• du bist = you are
• er/sie/es ist = he/she/it is

Since die Idee is singular, German uses ist.

Prima is a common German adjective meaning something like:

• great
• excellent
• really good

It is a normal, idiomatic German word.

Compared with gut:

• gut = good
• prima = great / excellent / very good

So Die Idee ist prima sounds a bit more enthusiastic than Die Idee ist gut.

Because prima comes after the verb sein and is being used as a predicate adjective.

In German, adjectives used this way usually do not take endings:

• Die Idee ist gut.
• Das Buch ist interessant.
• Die Kinder sind müde.

So here, prima stays just prima.

Yes. If the adjective comes before a noun, it usually takes an ending.

Compare:

• Die Idee ist prima.
  Here prima is after ist, so no ending.

• eine prima Idee
  Here prima is before Idee, so it is part of the noun phrase.

A useful thing to know here is that prima often stays prima even before a noun as well, because it is commonly treated as an indeclinable adjective in everyday use:

• eine prima Idee
• ein prima Plan

So the important beginner rule is:

• after sein: no adjective ending
• before a noun: adjectives often take endings, though prima is a special word that often stays unchanged

Normally, no. In standard German, singular countable nouns usually need an article or some other determiner.

So you would usually say:

• Die Idee ist prima. = The idea is great.
• Eine Idee ist prima. = An idea is great.

Leaving out the article would sound incomplete in most normal contexts.

A rough English-friendly guide:

• Die ≈ dee
• Idee ≈ ee-DAY
• ist ≈ ist
• prima ≈ PREE-ma

A few notes:

• die is pronounced like the English word dee, not like English die
• In Idee, the stress is on the second syllable
• In prima, the stress is on the first syllable

Prima is common and natural, but it sounds a bit more everyday and enthusiastic than a neutral adjective like gut.

For example:

• Die Idee ist gut. = neutral
• Die Idee ist prima. = more positive, lively

It is not slang, but it is slightly more conversational in tone.

In a yes/no question, German usually puts the verb first:

• Ist die Idee prima? = Is the idea great?

So the statement:

• Die Idee ist prima.

becomes the question:

• Ist die Idee prima?

Yes. That is a very common sentence.

• Die Idee ist prima. = The idea is great.
• Das ist eine prima Idee. = That is a great idea.

The second version is often used when reacting to a suggestion someone has just made.