i don't think he had a good time ,did he?为什么用"did he"

当陈述部分中主句的主语为第一人称，且动词为think，believe等词时，附加疑问部分与从句保持一致；另外，主句中don't think是从句的否定转移，因此，后面反义疑问句中的附加部分用肯定形式。

Since it is related to: he had a good time..."

因为后面的反义疑问句是针对he had a good time来进行提问的，所以时态应该保持一致。

因为前半部分是否定的，相当于：

He didn't have a good time, did he?

有些动词否定前置，如THINK，这类句子否定时要看从句。
上句等同于I think he did not have a good time, did he?

前否后肯