than是介词吗？可查的英汉词典上是介词呢？

可以是介词也可以是连词
Tom is taller than him.(than做介词)
Tom is taller than he.(than做连词)

是介词

是介词!!用于比较级!!

是连词

[用于形容词、副词比较级之后]比较; 比
He is taller than his brother.
他比他的兄弟长得高
。
[用于rather, sooner, 之后]与其...(宁愿...)
I'd rather you came tomorrow than today.
我宁愿你于明天来而不是今天来。

[用于 other, else, anywhere 等之后]除...以外
They were given nothing other than dry bread and water for their evening meal.
他们晚餐仅仅分到干面包和水。

[用于scarcely, hardly之后表示时间](刚刚...)就

Hardly had I finished telling the news than she began to cry.
我还没有把消息讲完她就哭了起来。