1. Introductory words, phrases and clauses are followed by a comma.
Moreover is an introductory word and should be followed by a comma.
“To become fluent readers” is an introductory infinitive phrase and should be followed by a comma.
“If you want to write well” is an introductory clause and should be followed by a comma.
2. Nonessential information is set off with commas.
“Who was born in Poland” is a clause that provides nonessential information and should be set off by commas.
3. Essential information does not require commas.
“Who vandalized the public gardens” is essential information because it identifies which boys are meant. It should not be set off by commas.
4. A comma is placed before a coordinate conjunction that joins two independent clauses.
The clauses joined by the conjunction but could stand alone as complete sentences: “The cougar moved quickly” and “The tourist reached the safety of the cabin.” A comma is not needed with a compound verb joined by a coordinate conjunction: “The tourist saw the cougar and ran to the cabin.”
5. A comma is not strong enough to join two main clauses (comma splice).