go in for politics;He went in by the back door

问题描述:

go in for politics;He went in by the back door
go in是“进去,进入”,可我觉得没有介词for和by也解释的通啊.
请问向这种情况我怎么能意识到要加介词

读出来看句子顺不顺
例如;
he went in the back door
他走进后门
he went in by the back door
他从后门走进去
你认为,哪一个比较好?