1. the subject of theoretical grammar its relations to other branches of linguistics

3. the notion of grammatical forms, category and opposition.
There are 3 fundamental notions: grammatical form, grammatical meaning, and grammatical category. Notional words possess some morphemic features expressing grammatical meanings. They determine the grammatical form of the word.
Grammatical form is not confined to an individual meaning of the word because grammatical meaning is very abstract & general ex: oats-wheat: The grammatical form of oats is clearly plural and grammatical form of wheat is singular, but we can’t say that oats are more than one& wheat is one. So here we say that oats is grammatical. Plural & wheat is grammatical singular. There is no clear one-to-one correspondence between grammatical category of singular & plural and counting them in reality in terms of “one” and “more than one”.
A very vivid example confirming the rightness of this statement is connected with the category of gender with biological sex ex: bull-cow, so the grammatical form presents a division of a word of the principle of expressing a certain grammatical. meaning.
Grammatical meaning is very abstractive generalized meaning, which is linguistically expressed. ex: Peter’s head -the grammatical meaning of the category of case showing the relations between part and a whole.
Grammatical meaning is always expressed either explicitly or implicitly. For instance: The book reads well here the grammatical. meaning of passivity is expressed implicitly.
Grammatical meaning is a system of expressing the grammatical meaning through the paradigmatic correlation of grammatical forms-expressed by grammatical opposition, which can be of different types:
Private
Gradual-large-larger-largest
Equipollent-am is are
Any grammatical category must be represented by at least two grammatical forms (e.g. the grammatical category of number – singular and plural forms). The relation between two grammatical forms differing in meaning and external signs is called opposition – book::books (unmarked member/marked member). All grammatical categories find their realization through oppositions, e.g. the grammatical category of number is realized through the opposition singular::plural.
Taking all the above mentioned into consideration, we may define the grammatical category as the opposition between two mutually exclusive form-classes (a form-class is a set of words with the same explicit grammatical meaning).