Following Passag

Grammar is the field of of linguistics that covers the conventions governing the use of any given natural language. it includes morphology an syntax, often complemented by phonetics, phonology semantics, and pragmatics. 

Each language itself own distinct grammar.  "English grammar" is the set of rules of the English language itself. "an English grammar" is a specific study r analysis of these rules.  A reference book describing the grammar of a language is called a " reference grammar" or simply "A grammar"