should be " I can't believe how much you'VE CHANGED since the last time I SAW you !"

WOW that Canadian guy can't speak English! Must be French-Canadian. Cynthia has the right answer. " I can't believe how much you'VE CHANGED since the last time I SAW you !" Better blacklist Denis here.

Honey, There are four possibilities. 1) I can't believe how much you've changed since the last time I saw you ! This is the grammatically correct version if you are taking an ESL test. "Since" usually requires the simple past when referring to a definite time (the last time). A perfect tense is used in the other clause because it expresses an indefinite amount of time since the last time you saw him. In this sentence you are thinking about how the person looked at the moment you last saw him. 2) I can't believe how much you've changed since the last time I've seen you ! This is a bit odd because when you mention a definite time in the past (the last time) you usually don't use the present perfect. You often hear this in spoken English. Here you are thinking about the amount of time passed since, not the act of seeing him. Perhaps, you don't remember exactly when the last time was that you saw him, so for you it is not definite. In my opinion this sentence is grammatically correct. 3) I can't believe how much you changed since the last time I saw you ! This is technically not correct, but because the verb "to change" is intransitive here and describes a process that can go on for an indefinite period of time, the simple past of a verb like 'changed' is often used in spoken or informal American English, instead of the present perfect. 4) I can't believe how much you changed since the last time I have seen you ! This is another version of 3) in which the amount of time since you have last seen him is stressed rather than the moment when you actually did see him.

Hello Honey, Here you use the Present Perfect 'have changed'. This tense is used in many cases to express an action that started in the past and is still existent in the present. The change happened in the near past probably and of course is present at the moment when the speaker expresses it. The usage of 'since' applies as well using the Present Perfect tense.

I can't believe how much you've changed since the last time I saw you !!!!!!!!!! --- correct tense....