1 Question: Why is it necessary to follow a Mujtahid?
Answer: It is necessary for a Muslim to believe in the fundamentals of faith with his own insight and understanding, and he cannot follow anyone in this respect i.e. he cannot accept the word of another who knows, simply because he has said it. However, one who has faith in the true tenets of Islam, and manifests it by his deeds, is a Muslim and Mo'min, even if he is not very profound, and the laws related to a Muslim will hold good for him. In matters of religious laws, apart from the ones clearly defined, or ones which are indisputable, a person must act according to the verdicts (Fatwa) of the most learned Mujtahid.
2 Question: Why do Shias use Turbah (Sajdagah) when doing Sajda?
Answer: Sajdah should be performed on earth, and on those things which are not edible nor worn, and on things which grow from earth (e.g. wood and leaves of trees). It is not permissible to perform Sajdah on things which are used as food or dress (e.g. wheat, barley and cotton etc.), or on things which are not considered to be parts of the earth (e.g. gold, silver, etc.). And in the situation of helplessness, asphalt and tar will have preference over other non-allowable things. Sajdah should not be performed on the vine leaves, when they are delicate and hence edible. Otherwise, there is no objection. It is in order to perform Sajdah on things which grow from the earth, and serve as fodder for animals (e.g. grass, hay etc.). It is in order to perform Sajdah on flowers which are not edible, and also on medicinal herbs which grow from the earth.
3 Question: Why do Shias combine Zuhr and Asr prayers? Is it permissible?
Answer: It is permissible to combine Zuhr and Asr prayers and to perform them separately as it is permissible in both manners. It is narrated that the Holy Prophet of Islam (pbuh) offered his prayers in both combined as well separate manners. He combined his prayers without being in an urgent situation like war or storm.