What is an easy way to teach present perfect tense so my ESL students can understand it?
Present Perfect is used to indicate past events which have influence on present situation so to point the difference between this tense and Past Simple tense you can use the following example (from 'English Grammar in Use' by R. Murphy)
Study the difference in meaning between:
I lost my keys.
I have lost my keys.
The first sentence tells only what happened in the past. We do not know when was it and if the speaker found the keys later or not.
The second sentence uses the Present Perfect tense, so there is a connection between past event and present situation. I have lost my keys means that now I don't have them.
Note also that there is a difference in time description in those tenses.
Past Simple can tell about a speciffied time in the past. It is OK to say Yesterday I saw a good film in the cinema.
While Present Perfect tells about undefined time in the (near) past. You cannot say Yesterday I have seen a good film, but it is correct to use word like recently here.