You can pray Dhuhr and Asr or Maghrib and `Isha’ together in these cases: for the traveller, for one who is sick, for one who is breastfeeding, one who is unable to purify himself with water or to do Tayammum for every prayer, one who is unable to work out the time, and one who is suffering from Istihadah.

If you are travelling and you prayed `Isha’ before Maghrib, your `Isha’ prayer is not valid because you offered it before Maghrib and offering the prayers in the correct order is a condition of combining prayer when travelling.

The distance which allows a traveller to break his fasting and shorten his prayers is approximately 80 km according to the view of the majority of scholars. This distance is called travelling according to the custom of the Muslims. So if a person travels by camel, car, plane or ship, for this distance or more, he is regarded as a traveller.

If the sick person cannot do Wudu with water because he is unable to or because he is afraid that it will make his sickness worse or delay his recovery, then he should do Tayammum. If the sick people cannot stand, then he may pray sitting. If he is unable to pray sitting, he may pray lying on his side. If he is unable to pray on his side, he may pray lying on his back with his feet towards the Qiblah.

If the worshipper is able to stand and lie down, but he is not able to sit, then he may pray standing, leaning forward for bowing and prostrating, and reciting the Tashahhud whilst standing, then saying the Taslim. If there is a sick person who can either stand for the entire prayer or sit for the entire prayer, then he should pray sitting.

1. It is obligatory for the Imam to protect the prayer of those who are praying behind him from becoming invalid and to remember the number of Rak`ahs; he should not peck in the prayer (i.e., move too quickly), which undermines the pillars of the prayer, and he should not fall short in fulfilling the conditions of the prayer; he should make sure that he does the Sunnahs and postures of prayer properly, and so on. 2. The Imam takes on behalf of those who are praying behind him the responsibility of reciting out loud in the prayers where recitation is to be done out loud, and the responsibility of reciting the short Surahs (after Al-Fatihah) too. He also covers the mistake of the one who prays behind him if he omits some of the Sunnahs and even if he omits reciting Al-Fatihah if he comes late to the prayer. 3. The Imam is responsible for offering supplication for all those who pray behind him if he says Qunut or offers supplication for them. He is responsible for teaching those who prayed behind him the rulings on prayer so that it will not be spoiled and they will not be deprived of the full reward.

If the Imam’s mistakes do not alter the meaning of Al-Fatihah, there is nothing wrong with praying behind him. But if his mistake changes the meaning, then people should not pray behind him if he does not benefit from prompting.

1- When a person prays behind an imam, four scenarios may apply: 1- doing a movement before the imam, 2- delaying in following the imam, 3- doing an action simultaneously with the imam, and 4- doing a movement immediately after the imam. 2- Following the imam is the Sunnah and what it means is that a person starts to do the actions of prayer immediately after the imam starts them, but without doing them at the same time.

The one who most deserves to lead the prayer is the one who has the most knowledge of the rulings on prayer and has memorized the most Quran.