3 Word classes

1 There are different classes of word, somtimes called 'parts of speech'. The word
come is a verb, letter is a noun and great is an adjective.
NOTE
  Some words belong to more than one word class, For example, test can be a noun or a verb.
     He passed the test. (noun)
     He had to test the machine. (verb)

2 There are eight main word classes in English.
Verb:                 climb, eat, welcome, be
Noun:                aircraft, country, lady, hour
Adjective:          good, British, cold, quick
Adverb:             quickly, always, approximately
Preposition:        to, of, at, on
Determiner:        the, his, some, forty-five
Pronoun:            we, you, them, myself
Conjunction:      and, but, so
NOTE There is also a small class of words called 'Interjection'. They include oh, ah and mhm.

3 Verbs, nouns, adjectives and adverbs are 'vocabulary words'. Learning vocabulary
means learning werbs, nouns, adjectives and adverbs.

Prepositions, determiners, pronouns and conjunctions belong to much smaller
classes. There words are sometimes called 'grammatical words'.

4 Most word classes can be divided into sub-classes. For example:
Verb                  Ordinary verb: go, like, think, apply
                          Auxiliary verb: is, had, can, must
Adverb              Adverb of manner: suddenly, quickly
                          Adverb of frequency: always, often
                          Adverb of place: there, nearby
                          Linking adverb: too, also
                          etc.
Determiner         Article: a, the
                          Possessive: my, his
                          Demonstrative: this, that
                          Quantifier: all, three