prayer times

Dhuhr                 After the sun’s zenith until late afternoon.

Asr                     From late afternoon until just before sunset.

Maghrib              From sunset until just before twilight

                         (when the sky turns completely dark).

Isha'                   From twilight until dawn.

All these prayers must be prayed at the times mentioned above. However, there are exceptions for when it is permissible to combine prayers.  For example, (Dhuhr and Asr) or (Maghrib and Isha') may be prayed right after each other.  You can combine these two prayers when you are traveling, if you are sick, very sleepy, very tired, or if you cannot control your schedule (classes or work).  These are the exceptions and should not become the norm. 

If you are praying at the Masjid (mosque) in a congregation, and there are bad weather conditions, like rain, snow, ice, etc., then it is permissible to combine these pairs of prayers.  You can pray the earlier prayer at the time of the later prayer or vice versa.