If you have an excuse and you were unable to pray on time, you must make the prayer up even if that is during one of the forbidden prayer times such as after the `Asr prayer.

If you miss a prayer, then you have to pray that prayer first, then pray the current prayer; it is not permissible to delay making up the missed prayer.

It is prescribed to make up the regular Sunnah prayers if they are missed because of an excuse which led to one missing them at the proper time, such as sleeping, forgetting or being too busy to do them. So, if you are unable to offer the regular Sunnah prayers before and after Dhuhr at the proper time, it is prescribed for you to offer them after `Asr.

Scholars differed with regard to making up Duha prayer. Some of them view that it should be made up, and this is the correct view according to the Shafi`is and some of the Hanbalis. Others view that it should not be made up.

It is obligatory to make up missed prayers in the proper order, according to the majority of scholars. Whoever misses Dhuhr and `Asr, for example, should pray Dhuhr first, then `Asr. However, if a person owes two prayers, Dhuhr and `Asr for example, and starts with `Asr by mistake or because he is unaware that the missed prayers should be offered in order, his prayer is valid.

It is not permissible to combine two prayers without a valid reason. If you put prayers together without any excuse or legitimate justification for doing so, then you are sinning.

For one whose regular practice is to pray for two hours at night, and he only prays half an hour because of some excuse, it is prescribed for him to make up the other hour and a half during the day.