An impersonal verb is a verb that does not have a true subject because it does not represent an action or state of being. Sentences that contain personal verbs typically have a subject noun — a person, place or thing that is acting in the sentence. Instead of a subject, a sentence with an impersonal verb usually starts with an impersonal or "dummy" pronoun, such as "it" or "there" in English. Impersonal verbs are found in many other languages with various other constructions.
Impersonal verbs are occasionally called "weather verbs" because of their frequent use in sentences about weather, such as "It snows regularly during the winter months in Michigan." In this example, "snows" is the impersonal verb, and "it" is the impersonal pronoun that functions as the grammatical subject of the sentence. Many Romance languages contain similar constructions, such as the French il neige, meaning "It snows."
In highly inflected languages such as Latin, impersonal verbs can be very common because inflected verbs do not necessarily require a subject. The simple verb amat, for instance, has the -at ending, which indicates that the subject is "he/she/it." No pronoun is required to make the subject clear. Similarly, the impersonal verb lecit, which means "it is permitted," does not require a subject independent of the verb. This construction does not have the dummy pronoun that can make impersonal verbs awkward in English.
Some rhetoricians recommend avoiding impersonal verbs and instead rewording a sentence to contain an active verb when possible. The sentence "There are six elephants at the zoo," for instance, can be rewritten as "Six elephants live at the zoo." Certain stock phrases, however, do not lend themselves to easy restructuring.
To determine whether an English sentence that starts with a pronoun contains an impersonal verb, one can look to see whether the pronoun has an antecedent. If it doesn't, it is likely an impersonal verb. The following set of examples makes this clear: "Mt. Kenya is 17,057 feet (5,199 m) tall. It has many species of wildlife," compared with: "Mt. Kenya is 17,057 feet (5,199 m) tall. It rains frequently there."
Although the two pairs of sentences seem similar on the surface, the first has an impersonal verb, but the second does not. The pronoun "it" in the first example has an antecedent: Mt. Kenya. The second example, however, has no antecedent — no previous noun to which refers. The mountain does not rain, it rains. The lack of an antecedent indicates that "rains" is an impersonal verb in this sentence.