"a frequent guest" "frequent complaints"
"a frequent (or common) error is using the transitive verb `lay' for the intransitive `lie'"
"She haunts the ballet"
误 The motoring accidents are frequent to happen at this crossroad.
正 The motoring accidents are frequent at this crossroad.
析frequent所表达的“频繁”这一概念,它本身已含有“发生”的意思,所以加用to happen这样一个不定式是多余的。