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

5 The notion of morph and allomorphs.
A morph is a phonological string (of phonemes) that cannot be broken down into smaller constituents that have a lexico-grammatical function. In some sense it corresponds to a word-form. An allomorph is a morph that has a unique set of grammatical or lexical features. All allomorphs with the same set of features forms a morpheme. A morpheme, then, is a set of allomorphs that have the same set of features. The morph 's' is linked to three distinct allomorphs, each containing a different set of features as indicated in the morpheme class: if it is adjoined to a noun, then it marks the plural; if it is adjoined to a verb, then it marks the third person singular of the verb; if it is adjoined to a noun phrase, then it it marks possession.
One way to represent a morpheme is by listing its features ([+Past]). Many linguists try to represent it by listing its chief allomorph if there are more than one allomorph ('s'). This is somewhat ambiguous in that "s" could stand for three morphemes, and is not a desirable way list a morpheme.
Each morpheme may have a different set of allomorphs. For example, "-en" is a second allomorph that marks plural in nouns (irregular, in only three known nouns: ox/ox+en, child/childr+en, brother/brether+en). The morph "-en" is linked to the allomorph "-en", which occurs in complementary distribution with "-s". When the possessive is adjoined to a noun phrase, there is only one phonological form, /s/, but it is written either as " 's " or " s'". The inflectional pattern of English pronouns is too complex to go into here. "-en" is a distinct morph from "s".
Morph is the phonetic realization of a morpheme which study the unit of form, sounds and phonetic symbol. The morphs can be devided into two important classes, lexical and grammatical.
Lexical morph is the morph that denote directly objects actions, qualities and other pieces of real word (ex : table, dog, walk, etc.)
Grammatical morph is the morph that has been modifiying the meaning of the lexical morphs by adding a certain element to them. (ex : un-, -able, re-, -d, in-, -ent, -ly, -al, -ize, -a-, -tion, anti-, dis-, -ment, -ari-, -an, -ism)
Allomorph is variant form of morpheme about the sounds and phonetic symbols but it doesn’t change the meaning. There are three types of allomorph, phonologically, morphologically and lexically conditioned allomorph.
So, allomorph is variant form of a morpheme about the sounds and phonetic symbol but it doesn’t change the meaning. Allomorph has different in pronounciation and spelling according to their condition. It means that allomorph will have different sound, pronounciation or spelling in different condition.