是everyone tries their best还是tries his/her best?

everyone has to try his best.

惯例是, 在不知道性别情况低下, 用 his

try one's best
不用tries吧

而且我觉得do好些
everyone‘s doing their best
或者
do her best
一般是用将来时 我会尽力的。
i will do my best

后面的 但是这里可以不用所有格