The perfect consists of the verb HAVE (i.e. a form of this verb) followed by the past participle (the third of the so called 'principal parts', as in go - went - gone). In the simplest case, a verb phrase in the perfect consists of two verbs only:
(1) Elvis has left the building.
However, both the first part of the perfect, i.e. the verb HAVE, and the second part, i.e. the past participle, may be part of another combination of verbs in the verb phrase. Thus, the first part of the perfect form may follow a modal auxiliary, as in the following example:
(2) Elvis could have left the building.
Moreover, the second part of the perfect may be the first part of the passive (enclosed in square brackets in the following example):
(3) Elvis has [been escorted] out of the building.
When the first part of the perfect is in the present tense (e.g. has left), the verb phrase is said to be in the present perfect. When the first part of the perfect is in the past tense (e.g. had left), the verb phrase is said to be in the past perfect.
Contrasting the present perfect and the simple past
Both the simple past tense and the present perfect are used to make reference to states, events, etc. in the past. They differ in whether they portray the past as related to the present or not.
Thus, the present perfect refers to something in the past that has a strong relevance to the present. For example, the perfect is used to describe a state that started in the past and continues into the present, as in the following example:
(4) Mary has been angry for two hours. (implication: She is still angry.)
In contrast, the simple past tense implies no connection to the present, so that (4) above contrasts with (5):
(5) Mary was angry for two hours. (implication: She is not angry now.)
In these contrasting examples, it is possible to use the same adverbial (for two hours) to indicate duration. Often, however, the simple past and the present perfect combine with different adverbials. For example, temporal prepositional phrases and dependent clauses introduced by since virtually always combine with the perfect:
(6) Mary has been angry since yesterday morning.
(7) Mary has been angry since she lost her wallet.
In contrast, the simple past co-occurs with time adverbials denoting a specific point or period in time, at or during which an event took place. In those cases the present perfect is generally unacceptable. The following examples illustrate this point (as usual, the symbol * precedes a sentence that is unacceptable):
(8a) I bought a new car yesterday.
(8b) *I have bought a new car yesterday.
(9a) I met my brother a month ago.
(9b) *I have met my brother a month ago.
(10a) I saw the play last week in London.
(10b) *I have seen the play last week in London)
Single events that took place in the past may, however, be related to the present in less direct ways. This is especially the case when no time adverbial is present to denote a specific time in the past. Consider the following examples:
(11) I have bought a new car.
(12) Somebody has eaten my porridge.
(13) All the ships have arrived safely.
In these examples, either a single event or a series of consecutive events has already happened at the time of utterance, and does not extend into the present.
Rather, it is the result of the event(s) that is relevant, and which the speaker chooses to focus on. This use of the present perfect is therefore often referred to as the resultative perfect.
Thus, in the first example, the relevance to the present lies in the speaker’s current ownership of a new car, in the others, it lies in the fact that there is no porridge left, and in the ships being at their destination.