I go swimming once a week go swimming動詞片語+頻率副詞once a week 那為什 I usually go fishing on weekends 頻率副詞可以在動詞片語前?

頻率副詞–大部分(不是絕對的定律)可置於動詞前，如你的例句 – I usually go fishing on weekends.
頻率副詞也可置句尾.
I go swimming once a week. 你的例句.
Friends visit them occasionally. – copied from Yahoo dictionary.
(頻率)副詞也可置句首.
Occasionally we go out to restaurants, but mostly we eat
at home. – copied from Longman.
在助動詞與主要動詞之間：I have rarely written to my brother without a good reason. – copied 自隨意窩.
文法只是整理出來的大原則 – 社會科學，但不是如數學物理等自然科學之鐵律.