Gree: Thanks, a standard expression of gratitude.
Gree: the double "e" indicates a strong "e" sound as is found in the words pea or free.
Shen: the "e" is pronounced in the same was as the word "then."
It should be noted that, unlike the English language where "please" can come at the beginning or end of a sentance such as in the following examples:
Please, buy milk.
Buy milk, please.
Shen should always come at the end of the sentance since it is an additive item to the main subject of the sentance.
Shen can stand alone in the assumptative response sense. For example:
[Do you want me to get milk?]
The "get milk" part of "Please get milk" is dropped in conversation but assumed to be the subject of the "please" given the context
Gree can come at any part of the sentance as an expression of gratitude is a standalone subject unto itself. In English you would have:
Thank you, I needed that.
I needed that, thank you.
Here the "for doing that" is dropped from the expression of gratitude and assumed, but it is still a different subject from the expression of need for the thing to have been done.
The best way to put it is that Gree, like "Thanks" in English can be it's own seperate sentance whereas Shen cannot except in the assumptative response described above.
You can say:
Thank you. That was nice of you. OR Thank you, that was nice of you.
You cannot say:
Please. Get me milk.
The correct form is:
Please, get me milk.