Grammar is the structure and system of a language, the structure of words (morphology) and sentences (syntax), and the study of structure of a language. In short, it is sometimes regarded as the "rules" of a language. Many native speakers speak their own language without having studied its grammar. Grammar helps students, especially foreign students, learn a language efficiently.
In speech and written forms of many speech languages, grammar is in its linear form, in which words and sentences are structured in a linear fashion. In written Chinese, Japanese and some other written languages, sentences are vertically linear but words are more of a non-linear form. The strokes of a character (Chinese or Japanese) are in an order but the spatial-visual appearance of a character is non-linear.
Visual-manual language (in its verbal modality) has both linear and non-linear forms. The grammar in Ameslan and other sign languages is linear in term of the grammatical structure (e.g. sequential order). It is also non-linear in term of using the three-dimensional space. Grammar in sign language has its "parts of speech", such as verbs (e.g. have, do, run), nouns (e.g. cat, music, hair), adjectives (e.g. big, blue, sticky), pronouns (e.g. you, ours, I), conjunctions (e.g. and, but, so), etc. How its parts of speech are formed is different in space from speech language.
Many English-speaking learners speak Ameslan with English grammar in it. E.g. Ameslan vocabulary using English grammar. It is known as Signed English. It is sometimes habitual for some English speakers to think English when speaking Amelsan. It is essential to study Ameslan (or other sign-language) grammar to understand and practice its structure.