Question

If only I__how to operate a computer as you do.

A.had known B.would know C.should know D.knew

Answer 1

D. （要是我能够像你一样知道如何操作电脑该多好啊）虚拟语气，
if only常常用来表达强烈的愿望或遗憾，因此，主要用在虚拟语气中，用以表达强烈的愿望或非真实条件。常被译为“但愿”、“要是……该多好啊”等。
几种时态的虚拟语气：我要是在这儿就好了。
现在时：If only I were here.
过去时：If only I had been here.
将来时：If only I would be here.

Answer 2

D.if only "要是。。。就好了"，后边句子用虚拟语气。整句话意思是：要是我能像你一样懂得操作电脑就好了。