小题1:He insists/insisted that he was at another place at the time of the crime .
小题2:It’s generous of her to devote most of her time to charity .
小题3:Mum was cooking dinner while I was doing my homework yesterday .
小题4:
I can’t decide whether to ask the teachers for help .
I can’t decide whether/if I should/will ask the teacher for help .
小题5:
He would rather keep silent than tell us the truth .
He prefers/preferred keeping silent to telling us the truth .
He prefers/preferred to keep silent rather than tell us the truth .