I often go fishing in the lake.为什么fish后面加ing不是go fishI

表示做某种活动时的固定用法,常用go doing sth. 表示.
做什么活动时 go to do 不常用.
这两者区别不大,都有“去做某事”的意思.
不过,go doing中的do一般都要求是不及物动词,如go shopping/swimming/walking等.
而go to do sth中的do无此要求,且有时可以转化成go and do sth.
如：Let me go to look at it. = Let me go and have a look at it.