Difference between "a good many" and "many a good"?

[Partha Roy]

English Language Tests, Intermediate level

ESL/EFL Test #838 "Real Life: Marriage (1)", question 1

An unmarried person is commonly called a ......... person.

(a) suitable
(b) friendly
(c) clear
(d) single

English Language Tests, Intermediate level

ESL/EFL Test #838 "Real Life: Marriage (1)", answer 1

An unmarried person is commonly called a single person.

Correct answer: (d) single
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what is the difference between " a good many" & " many a good"?

Partha Roy

[Mister Micawber]

a good many = many
many a good (+ noun) = many good (+ noun)
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Native English teacher at Mister Micawber's