yesterday的过去式是什么

yesterday是时间副词，也可用作名词，它没有过去式，它属于过去式的标志词。如：

I read an interesting book yesterday.我昨天看了一本很有意思的书。

He was playing basketball on yesterday morning.他昨天上午在打篮球。

He arrived in Singapore yesterday.他昨天到了新加坡。