youneedtorememberthatyourpurposeistounderstandandmakeyourselfunderstood.语句里的“tounderstand”为什么不能改成understand?todothisyoumustimitateohers'speakingatnormalspeed.语句里的“todo”为什么不能改

Question

you need to remember that your purpose is
to understand and make yourself understood.
语句里的“to understand”为什么不能改成understand?
to do this you must imitate ohers' speaking at normal speed.
语句里的“to do”为什么不能改成“do”?
while learning,try to concentrate on english sounds and setence ptterns instead of trying
to translate english into chinese sentence by sentence.
语句里为什么要用try的动名词形式trying?

Answer 1

is是be动词,后面不能直接加动词understand
第二个问题中的to do this可以把它看作状语,
动词只有在祈使句中才能直接使用
第三个问题中,instead of 中的of是介词,后面要跟动名词