先把这个句子补充完整:
What I thought I would do was buy a new car.
在这个句子中,what I thought I would do为嵌入式名词性关系分句。其中what为融合型关系代词 = the thing which。 所以,what I thought I would do = the thing which I thought I would do,其中which是I thought的宾语从句中的宾语。翻译为:,我认为我将要做的事情(就是买一辆新车)。