• basic subject: whether to use the present perfect or the simple past when referring to past events
• concentration: required use of the simple past in presence of an adverbial indicating the point in time at which something happened
• five of the ten items in the exercise require the simple past; in four of those, such an adverbial is actually present (and lightly highlighted); in the fifth it is implicit
• an important matter because a significant percentage of all verb tense errors: using present perfect with point-of-time adverbial
• to eliminate this error important to get students to understand the distinction between a point in time and a period of time. (in explaining this the ideas of "frame of reference" and "narrative" should be introduced)
• not easy because from the point of view of verb tense usage at any rate a particular point in time can be much larger than a particular period of time:
• compare the point in time adverbial "last year" as in "Sally went to Spain last year" with the period-of-time adverbial, "for five minutes" as in "Sally has been on the phone for five minutes."
• four of the five items which require the present perfect contain period-of-time adverbials (lightly highlighted); in the fifth of these items the present perfect is required, not because the event being referred to is located (explicitly or implicitly) in a period of time coming up to the present but because the event is being referred to because of its present consequences.
-fl,