Kiddushin 61 - How to make others fulfill your conditions
A natural condition may sound like, "If you do this, I will marry you." However, many things remain unsaid, such as what happens if he does not do this. To make your condition enforceable, you can follow these rules
* Condition must be "doubled," such as if yes, then.. but if no, then ...
* The stipulation must precede the act, that is, "If you do this, I will marry you..."
* The positive must precede the negative
Here is the model condition that Moses had set: "If the children of Gad and Reuven will cross the Jordan and help in the war, they will get the pastures they want" and "But if not, they will not get them." - so says Rabbi Meir.
However, Rabbi Chanina says that Moses had to mention that if they don't help they don't get the pastures in Transjordan, to make sure that they at least get their portion in Israel. That is why the "if no..." clause was needed, but typically it is not required.
Art: Torah Scribe by Artur Markowicz