your uncle has arrived here.i did not think he___until tomorrow为什么用was coming？

本来是 He is not coming until tomorrow.
瞬间动词come的进行时态,表示将来时含义, "他直到明天才会来".

因为是"我本以为...","在他到之前我还以为...",所以用一般过去时I thought,而后面的从句he is not coming要与之保持时态一致,所以改为he was not coming.

又因为 I think that句型, 如果后面从句是否定, 要把否定移到前面来.
因此本句从"I thought he was not coming..."变为 "I did not think he was coming..."

现在已经来了，我原以为他直到明天才来。

was coming 表过去将来时

进行时态表示将来，因为有did，所以用was.
你叔叔已经来到了，我不认为他直到明天才来。