STEP 1: DEFINITION

Word form in English refers to parts of speech and their usage. Words belong to families, and their forms must be carefully chosen. Nouns should be used as nouns, verbs should be used as verbs, adverbs should be used as adverbs and adjectives should be used as adjectives.

Here are three different word families:

NOUN VERB ADJECTIVE ADVERB
belief believe believable believably
obedience obey obediant obediently
predominance predominate predominant predominantly

PREFIXES AND SUFFIXES

Prefixes and suffixes can be added to root words to change their parts of speech and their meanings. For example, adding the prefix "un-" to the verb "do" creates the opposite meaning "undo." Adding the suffix "-ly" to the adjective "miserable" creates the adverb "miserably." 

Errors in word form occur when incorrect parts of speech are used.

Incorrect: Please take a deep breathe.

Correct: Please take a deep breath.

Incorrect: This change will have no affect on your decision.

Correct: This change will have no effect on your decision.

Incorrect: They have many believes.

Correct: They have many beliefs.

Incorrect: That statement is not necessary true.

Correct: That statement is not necessarily true.

Incorrect: My counselor gave me some advise.

Correct: My counselor gave me some advice.

STEP 2: PRACTICE EXERCISES

Practice 1 - Recognizing word form errors in sentences 
Practice 2 - Recognizing word form errors in paragraphs

STEP 3: LEARNING MORE

For more information on word form, follow these links:
http://ueno.cool.ne.jp/let/prefix.html