[Grammar] Would you explain me why these five sentences are not grammatical?

[eggcracker]

1.In the Middle Ages, there had been a few episode when popular imagination caused people to seek out someone who caused a flood.
2.The origin of the horse go back to eohippus, the "dawn-horse" of the Eocene, only 10 to 20 inches tall.
3.There are several kind of acids that pearls may come in contact with.
4.Dr. Rita Colwel is a microbiologist who wears several different hat.
5.In the early years of the twentieth century several man who had amassed vast fortunes likewise became great philanthropists.

I saw these wrong sentences from the book. But I don't know why these are not grammatical and there are no explanation in my book. Please give me some explanation about these.

 

[keannu]

1.In the Middle Ages, there had been a few episode(=>episodes) when popular imagination caused people to seek out someone who caused a flood.
=> "a few" goes with plural nouns as it's countable
2.The origin of the horse go(=)goes) back to eohippus, the "dawn-horse" of the Eocene, only 10 to 20 inches tall.
=>It should be "goes" as "origin" is an abstract noun that is inherently treated as singular
3.There are several kind(=>kinds) of acids that pearls may come in contact with.
=> "there are" goes with plural nouns
4.Dr. Rita Colwel is a microbiologist who wears several different hat.(hats)
=> "several" implies plural nouns
5.In the early years of the twentieth century several man(=>men) who had amassed vast fortunes likewise became great philanthropists.
=> same as 3

Not a teacher, but as a fellow Korean.