Because ryk is a predicate adjective: it describes the subject after the linking verb is.
Compare:
After is, the adjective stays in its basic form.
Before a noun, Afrikaans adjectives often change form, often by adding -e.
Usually yes. Man means man, an adult male person.
In some contexts, man can also mean husband, just like English sometimes does, but in this sentence it simply means man.
Also note the plural:
So Afrikaans does not use an irregular plural like English men.
You say:
Afrikaans usually uses a double nie in simple negatives:
So the pattern is often:
subject + verb + nie + rest + nie
For a yes/no question, Afrikaans usually puts the verb first:
Is die man ryk?
= Is the man rich?
So the statement:
becomes the question:
No. Afrikaans is much simpler than many European languages in this respect.
In a sentence like Die man is ryk:
So you can focus on the basic sentence pattern without worrying about gender agreement.