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

4, 8. Grammatical homonymy in morphology and syntax.
Morphological synonymy reflects a variety of representations by different parts of speech for the same meaning, e.g. due to (adjective), thanks to (noun), because of (preposition), etc.
Morphological homonymy may be described as phonetic equivalents with different grammatical functions, e.g. He looks – her looks; they wanted – the job wanted; smoking is harmful – a smoking man; you read – we saw you, etc.
Two or more morphemes may sound the same but be basically different, that is, they may be homonyms. Thus the -er morpheme indicating the doer of an action as in writer has a homonym — the morpheme -er denoting the comparative degree of adjectives and adverbs, as in longer. Which of the two homonymous morphemes is actually there in a given case can of course only be determined by examining the other morphemes in the word. Thus, the morpheme -er in our first example, writer, cannot possibly be the morpheme of the comparative degree, as the morpheme writ- to which it is joined on is not the stem of an adjective or adverb, and so no comparative degree is to be thought of here.
In cases of polysemy and homonymy, two or more units of the plane of content correspond to one unit of the plane of expression. For instance, the verbal form of the present indefinite (one unit in the plane of expression) polysemantically renders the grammatical meanings of habitual action, action at the present moment, action taken as a general truth (several units in the plane of content). The morphemic material element -s/-es (in pronunciation [-s, -z, -iz]), i.e. one unit in the plane of expression (in so far as the functional semantics of the elements is common to all of them indiscriminately), homonymically renders the grammatical meanings of the third person singular of the verbal present tense, the plural of the noun, the possessive form of the noun, i.e. several units of the plane of content.
In cases of synonymy, conversely, two or more units of the plane of expression correspond to one unit of the plane
of content. For instance, the forms of the verbal future indefinite, future continuous, and present continuous (several units in the plane of expression) can in certain contexts synonymically render the meaning of a future action (one unit in the plane of content).