I find it is always helpful to have students review a grammar point before class, so provide your students with a short explanation of both present perfect and simple past tense and the distinction between the two. If you have a good one in their native language, at this level, it's OK to use it. If you prefer to use an English explanation, you can find a short, simple one here. For an in depth explanation of perfect tenses, see here.
The key distinction between present perfect and simple past is that present perfect is used to talk about past events that are relevant to the present. This could be for several reasons.
- The time period, either concrete or implied, that is being discussed is not finished.
I have seen that guy three times today. (Today has not finished, so it is possible you may see him again before the day is through.)
They have checked the mailbox five times already. (Implies they will keep checking until the mail arrives)
- The fact that something happened in the past is being related to convey a present level of experience or completion.
She has been to the Pyramids. (Relevant to the present because it conveys that she holds a certain experience.)
He has finished the report. (Relevant to the present because it conveys that a task is currently complete.)
- Something has changed between a point in the past and the present moment.
You've grown about a foot since I last saw you!