1). What is a "cognate"?

Question

2). Why is learning cognates the most efficient and rapid way to learn to read, speak, increase vocabularly and get high marks on tests in a foreign language?

Bonus question: What is the relationship between "cognates" and "etymology"?

Definition of ETYMOLOGY

1
: the history of a linguistic form (as a word) shown by tracing its development since its earliest recorded occurrence in the language where it is found, by tracing its transmission from one language to another, by analyzing it into its component parts, by identifying its cognates in other languages, or by tracing it and its cognates to a common ancestral form in an ancestral language.

Answer 1

http://dictionary.reference.com/browse/cognate