■若其中的动词be为现在式(am, is, are),则其后不定式通常只能是静态动词,不能是动态动词。如:
They are fortunate to be able to buy such a large a house. 他们真幸运,能买那样大的房子。
但不能说:They are fortunate to buy such a large house.
■若要在am [is, are] fortunate后接动态动词,则要用完成式或进行式。如:
They are fortunate to have bought such a large house. 他们很幸运,买了那样大的房子。
You are fortunate to be working in London. 你真是幸运,能在伦敦工作。
■若其中的动词be为过去式(was, were),则其后不定式便没有以上限制。