Furor regis magnus est.

Magnus has to agree with furor, the noun it describes. Since furor is masculine nominative singular, the adjective must also be masculine nominative singular: magnus.

If the noun were neuter, you might expect magnum. If it were feminine, you might expect magna.

Est is the third-person singular present form of esse, meaning to be. It means is.

So the basic structure is:

• furor = fury
• magnus = great
• est = is

Together: The king’s fury is great.

Latin word order is more flexible than English word order because the endings show each word’s role in the sentence.

A very literal word-for-word order here is:

• furor = fury
• regis = of the king
• magnus = great
• est = is

So literally: Fury of the king great is.

That sounds strange in English, but it is normal in Latin. The endings, not the position alone, tell you how the words fit together.

It describes furor, not regis.

You can tell because magnus is nominative singular masculine, matching furor. But regis is genitive singular, so magnus does not agree with it.

In Latin, adjectives normally agree with the nouns they modify in gender, number, and case.

Yes. This is a standard use of the genitive case called the genitive of possession. It shows whose fury it is: the king’s.

So:

• furor regis = the fury of the king
• more naturally in English: the king’s fury

Yes, often it could. For example, Latin could also say:

• Magnus furor regis est
• Furor magnus regis est

The exact word order can affect emphasis or style, but the basic meaning remains the same because the forms of the words still show their grammatical function.

You would usually look them up as:

• furor, furoris = fury, madness
• rex, regis = king
• magnus, magna, magnum = great
• sum, esse = to be

This is useful because Latin words change form, and the dictionary gives you the base form plus enough information to recognize other forms like regis or est.