As if it were not already complicated enough, English contains a group of words that look like verbs and function as verbs in other contexts but are not always used as verbs.  This group of words comprises verbals – gerunds, infinitives, and participles.  They look like verbs but do not function as verbs.

• Gerunds are verb forms that function as nouns.  We all know that, in "I am writing an essay," writing is part of the verb am writing.  It describes an action.  But what about "Writing can be a challenge"?  The word writing refers to the act of writing; it is the subject of the verb can be.  Hence, a word that we usually consider to be a verb denoting an action is used as a noun denoting the act.  Gerunds always end in –ing. 
  
  
• Infinitives are verb forms that function as nouns and sometimes as modifiers.  The infinitive consists of the base form of the verb preceded by the word to.  In "To err is human," the infinitive To err is an infinitive and serves as the subject of the sentence.  In "To achieve our goals, we work hard," To achieve our goals is an infinitive phrase modifying the verb work (why we work), so it functions as an adverb or an adverb phrase.  In "I often have an impulse to shop," to shop modifies the noun impulse (what kind of impulse), so it functions as an adjective or an adjective phrase.
  
  
• Participles are verb forms that function as adjectives.  Present participles end in –ing; past participles end in –d or –ed.  When we speak of "barking dogs," we are using the present participle of the verb bark as an adjective modifying dogs.  When we speak of "shattered dreams," we are using the past participle of the verb shatter to modify dreams.