An impersonal verb is one which has no clear semantic Subject. In English, the grammatical Subject is usually an impersonal pronoun - the word it, which is used with the third person singular of the present tense. This gives such sentences as "It is raining", where the grammatical function of the word it is clear, but there is no phenomenon in the real world to which it refers.
Impersonal verbs are also currently used in such constructions as "It seems to me...", "it occurs to me", in which an essentially subjective experience is expressed in quasi-objective terms. It used to have a wider range, and in Old English a more recognized grammatical status. It was applied with
- certain verbs of liking or recommending, etc. Some still survive in formal language: it is proper to start a petition to the Queen with the phrasal impersonal construction "May it please the Queen's most excellent Majesty..." To describe someone's whim, an historian may write "It pleased her to ..."
- Birchfield's Fowler says that impersonal verbs were "once a rich class of verbs originally (in O[ld] E[nglish]]]) governing either an accusative or dative case. Examples of past uses: What boots it with incessant care To tend the homely slighted shepherd's trade...? (Milton) 'What good does it do ...?); him listeth 'he is pleased ...; methinks 'it seems to me'". (the last two are common in Shakespeare and his contemporaries.)
- This is akin to their use in Latin and other Romance languages, along with many European and non-European tongues, where the grammatical Subject of a verb may be expressed purely by inflection, with no separate Subject word. In Latin, for example, one way of saying 'I ought to ...' was me opportet, literally 'me [it] behoves', or '[it] is incumbent [on] me', where the English pronoun 'it' (and the continuous aspect 'is') is expressed by the inflection -et and the preposition 'on' is expressed by the Latin case-ending of me, the accusative of ego, 'I'. Similarly, a preference could be expressed by the two words Mihi placet: '[it is] pleasing [to] me', using the impersonal verb placet and mihi, the dative case of ego. Shakespeare and other Early Modern English writers could still say "it likes him not", which similarly has a personal pronoun in the Objective case, as an alternative to 'he doesn't like it'.