If I knew that I would never have told him.

▲网友的句子可以用过去完成时，而且通常应该如此：

If I had known that I would never have come. 我如果早知道的话，我是绝不会来的。

▲比如下面的句子，if虚拟条件句均是针对过去情况，均用的是过去完成时：

If I had known，I’d have told you. 如果我知道，我就告诉你了。

I could have warned you if I had known where you were. 我要是知道你在哪里的话就提醒你了。

I might have called if I had known you were here. 假如我知道你在这儿，我或许就会打电话了。

If I had known that you were coming I would have met you at the air-port. 要是我早知道你要来的话，我就会在机场接你了。

I wouldn’t have been so depressed if I had known how common this feeling is. 如果早知道这种感觉如此普遍，我就不会那么沮丧了。

If I had known you were in the office building, I would have looked you up. 如果知道你在办公楼里，我就来找你了。

▲但是不是网友的句子用一般过去时就错了呢？也不是！因为在现代英语中，有时也会在if从句该用过去完成时的地方使用一般过去时，这种用法在美国英语中尤其常见。请看 Swan 在其 PEU 中的相关说明：