Look at your text on page 26 What tense is the verb in each sentence in italic? Present perfect Is the action or situation in each sentence finished? No Which sentence focuses on a situation up to now? Cellphones have been around
Present Perfect We use it to talk about actions or states that began in the past and are continuing now.
Difference between simple past and present perfect Do not use the present perfect when you want to talk about a finished time in the past like yesterday or three days ago. Use the past simple instead. The present perfect is generally used when we are giving recent news, and there is a connection with the present. On the contrary, we use the simple past to give older information: we are only talking about the past.
He has failed his driving test again. (This is new information) He failed his exam two times last year (This is old information) I have been ill all the week. (There is a connection with the present) I was very ill last summer. I had 'flu. (No connection with the present)
Let’s see if you understand the difference http://www.eflnet.com/grammar/presperf1.php http://esl.about.com/library/quiz/bl_pastorperfect1.htm?lastQuestion=0&answers=0&submit=Next+Question+%3E%3E&ccount=0
In present perfect we can use: since / for for + period A period is a duration of time, for example: 5 minutes, 2 weeks, 6 years. For means "from the beginning of the period until the end of the period." For can be used with all tenses. since + point A point is a precise moment in time, for example: 9 o'clock, 1st January, Monday. Since means "from a point in the past until now." Since is normally used with perfect tenses.