A. Can or Could
1. In my first week as a sophomore, i almost lost my self confidence because [i can't] catch up with my classmates' learning pace.
2. i tried very hard to talk to him but i [couldn't] change his mind.
3. i was the only one who [can] understand you, but now i [couldn't] either.
4. coach saw my dance move and he said I [could] join the crew for the upcoming dance contest.
5. Good morning ma'am!, [can] you please check if this plan is fine with you?
6. if she practice more, she [could] sing beautifully.
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B. Will or Would
1. I'm not sure but Sam said he [will], come.
2. Marco, [will] you please erase the board for me?
