1. That is how i did so good. :(
2. That was how i did so good. :(
==> That is (the reason) why I did so well. :D
==> That was (the reason) why I did so well. :D
3. Were you supposed to work today? :D
4. Were you suppose to work today? :(
==> supposed to is a set phrase. :D
Excellent! Yes. That's a good test for grammaticality. :D 8)Is it b/c if I change it to sentence form, it is "You were supposed to work today." ?
5. I think he saw me. :D
6. I think he sees me. :D
==> I think THIS. (THIS can be past, present or future.) e.g., I think he will see me. (OK)
7. I thought he saw me. :D
8. I thought he sees me. :(
==> thought is past tense, so 'see' needs to be past as well;
I thought he saw me.
I thought he had seen me.
I though he would see me.
I thought he could see me.
All the best, :D