Can 'many' + singular noun be grammatically correct? (many a teacher)

[Schwartz]

-In American schools many a teacher begins the day by greeting the children, taking attendance, and saluting the flag.
Why don't we use 'many teachers' ?
Can many+ singular noun. be grammatically correct?

-What's more important is that you can practise them by using the 'trial and error method'.
Why could this clause have more than one 'verb to be' ?

[Morteza]

Hi,
1)"Many a + singular nons + singular verbs" is used in a formal/old-fashioned English.
It means "A lot of people/things" :
Many a famous pop star has been ruined by illegal drugs.
2)''What's more important"- a noun clause- is considered as a subject here,so after subjects we need verbs:What you did was wrong.
Hope this helps!
Morteza

[Schwartz]

Thank you.