Proprt use of words.
Proper use of words.
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What does lexicology include?
Affixes (affixes include prefix, suffix, infix, diminutive, tmesis).
Classes of words (parts of speech).
Complex words, compound words. 
Derivation. 
Inflections.
Idiomatic combinations.
Phrases.
Synonyms and antonyms.
Proper use of words.
Words with multiple meanings.
Word meaning (semantics).

Do you think anything needs to be added?