If you have vs if you had Is there a way to say 


  if you had come to the meeting... ,


in other words the untrue condition, as opposed to :


  if you have come to the meeting... ,


the mere if clause? I used to think it was 假如, but dictionary examples suggest otherwise. For example:


  假如有這必要, 就這麼幹吧。


EDIT: 

If you use context to express this difference, please provide examples!
I would really appreciate an explanation in English (apart from the examples). My Chinese is basic and if I have to translate I only get the basics.
you may consider "倘若", "假如", or "假使"

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000051670

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000089761

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000089751

but, the chinese language is "fuzzy", these terms do not absolutely indicate an untrue condition in the clause; they just merely imply such.