[Lingo] verb-iru

[steve smith <sjs@example.com>]

Hi

I'm sure I'll ask this badly because I don't quite know what I'm asking. 
 I've noticed that some verbs just don't show up without iru and I 
don't quite understand what is going on.

An example would be 知る.  You would ask 「知っていますか」 and not 「知 
りますか」, right?  Same goes for 立つ、混む、座る、曇る、晴れる and I'm 
sure other verbs.  Why do you use the 'ている' for these and not for 
others?  What makes these special and how can I spot them?  Does this 
question make sense or have I wandered off on some tangent?

Thanks!

Steve S.