A lot of discussions on how to learn languages mention grammar – whether it should be learnt overtly at all; whether it should be introduced gradually from the start, or only after one has a some knowledge of the new language, and so on.
There are often asides about how English-speaking people, especially the younger generations of English speakers, don’t even know the grammar of their own language.
What people mean by grammar is rarely discussed or defined, as it is assumed that everyone knows what grammar is, don’t they?
The OED has the following on grammar:
“That department of the study of a language which deals with its inflexional forms or other means of indicating the relations of words in the sentence, and with the rules for employing these in accordance with established usage; usually including also the department which deals with the phonetic system of the language and the principles of its representation in writing.
In early English use grammar meant only Latin grammar, as Latin was the only language that was taught grammatically. In the 16th century there are some traces of a perception that the word might have an extended application to other languages; but it was not before the 17th century that it became so completely a generic term that there was any need to speak explicitly of ‘Latin grammar’. Ben Jonson’s book, written c1600, was applied the first to treat of ‘English grammar’ under that name.
As above defined, grammar is a body of statements of fact — a ‘science’; but a large portion of it may be viewed as consisting of rules for practice, and so as forming an ‘art’. The old-fashioned definition of grammar as ‘the art of speaking and writing a language correctly’ is from the modern point of view in one respect too narrow, because it applies only to a portion of this branch of study; in another respect, it is too wide, and was so even from the older point of view, because many questions of ‘correctness’ in language were recognized as outside the province of grammar: e.g. the use of a word in a wrong sense, or a bad pronunciation or spelling, would not have been called a grammatical mistake. At the same time, it was and is customary, on grounds of convenience, for books professedly treating of grammar to include more or less information on points not strictly belonging to the subject.”
It seems that when people say that (other) English speakers don’t know their grammar, what they mean is that they might not be familiar with grammatical terms, such as subject, object, adverb, declension, etc, and/or that they do not always use standard language, or at least that they do not speak or write in the way that the critics believe they should.
In terms of language learning, grammar can refer to verb conjugations, noun declensions and other ways that words change to indicate such things as person, number, tense, mood, etc. So saying that Chinese ‘has no grammar’ indicates that it has no inflections.
What do you mean when you talk about grammar?
Grammar is a description of regularities and quasi-regularities in the morphology and syntax of a language.
Trying to recall how the term is used in my native Czech, I can’t remember any instances of Czechs saying about other Czechs that they don’t know their grammar if the criticised person departs from the standard idiom. In such situations the criticism is always put as “neumí (správně) česky” / “can’t speak (proper) Czech”. “Nezná gramatiku” would be naturally understood as if the person doesn’t know the terminology or so and this sort of remark is quite rare.
It seems to be more common to mention grammar if speaking about foreign languages, possibly because “grammar” is a fairly natural part of learning a foreign language since the very beginning and it thus feels like that one needs to know it to speak the language properly – while in the context of one’s mother tongue “grammar” is something one encounters at school when one already can speak the language fairly well, so “grammar” is perceived as a scientific description of the language not closely related to the ability to speak properly.