no it is not a correct sentence.
"He is good" is correct. 'He' is singular, 'are' is for plurals.
No, Are Jack and Jill good football players is
Always do good is correct and means what it says. "Always be a good doer" means something slightly different from what it says. A good doer is person who is known for doing good, not necessarily a person who does good.
One good workout for American football is doing pushups.
This sentence is completely correct.
A good sentence is when the grammar and spelling are correct and the idea you want to convey is clear.
The person's a criminal for doing what he did.
"he is a great chess player" is grammatically correct, but the rest could be better. No one really says footballer, a better choice would be Football player try: "he plays football and chess well." or "he's great at both football and chess."
When a book is good, I get lost. It was this sentence grammatical corrected.
Your brother is exceptionally good at football.
"My mathematics is good" is correct out of the choices given, but a better sentence would be "I am good at mathematics".