I became curious why in English grammar lacks an article in this Wikipedia page:
The subjunctive mood in English grammar includes particular verb forms that are used in certain clauses, chiefly dependent clauses, to express necessity, desire, purpose, suggestion and similar ideas, or a counterfactual condition.
I've found out that grammar in its "language structure" sense is described as a mass noun in the Oxford Dictionary, and this may be the reason.
But does it follow from this that we shouldn't use sentences like "these 2 languages have different grammars", since in its countable form the word means, according to the OD, "a book on grammar"? Is it necessary to keep grammar in the noncount form to carry this sense, say "these 2 languages differ in grammar" (or "their grammar structure")?