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The English vocabulary can be classified by different criteria and for different purposes.
Nouns，adjectives，verbs，numerals，and adverbs are full words or notional words，because they are meaningful in themselves，even if they appear in isolation. Full words function as independent members in sentences.
Articles，prepositions，conjunctions，and pronouns，pronominal adverbs are form words or functional words. They are meaningful only when used in conjunction with other words. So form words have no independent meaning proper.
Popular words are words usually used in ordinary conversation. Popular words are concerned with common things of life. They are a part of everyday vocabulary. They are also called informal words. So it is obvious that popular words belong to the people as a whole.
Learned words are words comparatively seldom used in ordinary conversation，but are used in books，newspapers，and documents. They are also called formal words. The meanings of learned words are familiar to every educated person，but there is little opportunity to use them at home. That is to say，learned words come from books we read，not from people's lips.
Abstract words are those which refer to states，events，or feelings，for examples difficulty，work，talk，meeting and thought.
Concrete words are those which are concerned with objects，and things，for examples，milk，beer，oil，and car.
How the majority of words were formed cannot be explained. These words are called opaque words such as：axe，book，table，and work.
Only a minority of words can be explained. They are words whose meaning is determined from the meaning of their components. These words are considered transparent words. Many echo-words and compound words are transparent ones，such as：cuckoo，ping-pong，motherland，and blackbird.
A polysemic word has a range of different meanings，while a monosemic word consists of a single meaning. In English，polysemy is the rule，but monosemy is the exception.
Words can be divided into native words and loan words according to their origin. Native words are Anglo-Saxon words still retained in Modern English，while loan words（borrowed words）are words taken from different foreign languages（曹磊，2003：9—10）.