Firstly Eid Mubarak to all the Muslims. I pray that Allah accepts our Fasting and gives us life to see and make the most of many more months of Ramadan.

Adey,

Certain rules in Islam are dependant on other things for them to come into force. For example; for Prayer; the position of the Sun determines the prayer. If the sun does not set or rise (as the case in certain geographies) then the prayer (whether the morning or evening prayer) is not applicable to those who live in such regions.In the absence of this other thing the rule does not come into force. Another example; sighting of the new moon determines the begining and end of Ramadan. If the moon is not spotted on the 29th then 30 days are completed. ( I won't get into hypothetics about what to do if the moon didn't exist as hypothetics are for those who like to waste their time). Lets stick to the reality and what affects us here and now.

There are many Sharia rules which are dependant on other causes for them to come into existence. then there are other rules which are conditional on other matters i.e. being in a state of purity is a condition of prayer but not the cause of prayer. Being sane, mature is a condition for liability in Islam...

There are other rules applicable to the sharia rules called preventors; for example the woman in menustration is prevented from praying or entering the mosque

There are such things called Licence--the traveller has a licence not to fast or he/she has a license to shorten his/her prayer.

Hope that clears some matters for you. There is a mountainous amount of detail and scholarly discussion and thoughts on the above within Islamic jurispudence.