1. Subject of theoretical grammar. Analytic and synthetic word forms

Synthetic Types
The number of morphemes used for deriving word-forms in Modern
English is very small (much smaller than either in German or in Russian,
for instance.
There is the ending -s (-es), with three variants of pronunciation and
the endings -en and -ren, in one or two words each, viz. oxen, brethren
(poet.), children.
There is the ending -'s, with the same three variants of pronunciation as
for the plural ending, used to form what is generally termed the genitive case of
nouns.
For adjectives, there are the endings -er and -est for the degrees of
comparison.
For verbs, there is the ending -s (-es) for the third person singular
present indicative, with the same three variants of pronunciation noted above
for nouns, the ending -d (-ed) for the past tense of certain verbs (with three
variants of pronunciation, again), the ending -d (ed) for the second participle
of certain verbs, the ending -n (-en) for the second participle of certain other
verbs, and the ending -ing for the first participle and also for the gerund.
Thus the total number of morphemes used to derive forms of words is
eleven or twelve, which is much less than the number found in languages
of a mainly synthetical structure.
It should also be noted that most of these endings are mono-semantic,
in the sense that they denote only one grammatical category and not two or
three (or more) at a time, as is the case in synthetic languages. For
example, the plural -s (or -es) denotes only the category of plural number, and
has nothing to do with any other grammatical category, such as case.
Sound Alternations
Sound alternations are a way of expressing grammatical categories which
consists in changing a sound inside the root. This method appears in Modern
English, for example, in nouns, as when the root vowel [au] of mouse is
changed into [aı] in mice, etc.
This method is much more extensively used in verbs, such as write —
wrote — written, sing — sang — sung, meet — met — met, etc. On the whole,
vowel alternation does play some part among the means of expressing
grammatical categories, though its part in Modern English has been much
reduced as compared to Old English.