When it says 7 more than a number equals 21, the "more than" is addition, a number is x (because it's the unknown number). So this one is x+7=21.
When it says 39 less than a number is 21. "Less than" means to subtract. The 39 is less than x, so it's x-39=21.
8 times a certain number increased by 1 is 65. Break it down, first 8 times a number, so it's 8*x or 8x. Then we add 1 to that, so it's 8x+1=65.
5 times some number decreased by 2 times the number is 42. Decreased by is subtraction. So it's 5x-2x=42.
When you look for the key words increased by (add), more than (add), multiplied by, times, less than (subtract), decreased by (subtract), is (equal sign), they will help determine what you need to do.
I hope this gives you a little more understanding. Good luck with it! :)