from time to time
at times；（every） now and then ；occasionally；once in a while有时；时时；偶尔；间或
I see him at the library from time to time．我有时在图书馆见到他。
He called at our home from time to time．他有时到我们家玩。
From time to time we invite experts to give lectures in our department ．我们经常请专家来我们系讲课。