两个意思都不一样

.

before是之前的意思

by有很多用法很多词组

具体事例具体分析

,

please

wash

your

hands

before

you

eat

.

请在吃东西前洗手

,

i

go

to

school

by

bus

.我坐巴士去上学.

the

window

was

broken

by

him.

窗户是被他打破的

.

等等

求采纳

!