The main verbal constructions used to express past time reference can be summarized briefly as follows:
1. The simple past expresses past states or events, e.g.:
He was here yesterday.
I saw him last night.
2. The present perfect, composed of have + -ed participle, expresses past states or events that have current relevance. It differs from the simple past in several ways, depending on the meaning of the particular verb:
continuation up to present vs. completion in the-past:
She has been here since three o'clock.
vs.
She was here from three till five
emphasis on present result of past event vs. past event itself:
She has cut her finger.
vs.
She cut her finger.
occurrence in the indefinite past vs. definite past:
She has visited the States several times.
vs.
She visited the States last year.
3. The perfect progressive, composed of have + been + -ing participle, like the present perfect, expresses states or events that started in the past and continue or are repeated up to the moment of speaking. The present progressive and present perfect are often interchangeable, but sometimes contrast clearly with each other, depending on the meaning of the particular verb:
incomplete vs. complete activity:
I've been writing a letter.
vs.
I've written a letter.
temporary vs. permanent state
I've been living here since 1977.
vs.
I've lived here since 1977.
4. Used to + infinitive refers to a past state or habitual event that took place in the indefinite past, and implies a contrast with the present situation, e.g.:
The pool used to be open until nine (i.e. but it isn't anymore).
I used to go swimming every day (i.e. but I don't anymore).
The errors described in 2.2.1 involve the misuse of the simple past for present time reference and, vice versa, the misuse of the simple present for past time reference. 2.2.2 deals with back-shifting or the sequence of tenses in reported speech.