Help to answer user questions in lessons

Here are the questions that our Russian language students ask on MR.com at the bottom of each lesson. These questions are closely related to the lessons, that's why they asked there rather than here on the forum. Feel free to follow the links below and answer the questions.

http://masterrussian.com/aa081201a.shtml#comments
Q: is there a way to know when "г" becomes "х" and not "к"? could it be that it becomes "x" after certain vowels that are stressed or unstressed? just taking a guess. Could I be wrong?

http://masterrussian.com/aa032301a.shtml#comments
Q: For "интернет", why isn't the Э there where it seems to be implied (with the bold letters in the pronunciation)? Because if the e is going to be replacing the Э, is that implied then? if so, I'm going to have a hard time understanding what's implied and what's not. Please Help!
or is it that in the middle of words, it uses the e instead of э?

http://masterrussian.com/blday_greeting.shtml#comments
Q: Translate: Have fun and be careful!