Shiism is a vast doctrine that includes many sub-sects. Some of these sub-sects are moderate and have slight differences from the Sunni doctrine. Others are so extreme in such a way that transgresses the basic principles of the Islamic belief insofar that the adherents of these extreme sects have stepped outside the fold of Islam.

With regard to offering prayer behind a Shiite, Dr. Hatem Al-Haj, professor of fiqh at the Shari`ah Academy of America, said,

The validity of praying behind an imam from a different sectdepends on whether this imam’s innovation amounts to disbelief.For the Shiites, this is still applicable. However, it is rather difficult tomake a blanket statement about all of them, because of the majordifferences among the different Shiite sects and even the differences within the same sub-sect.

Those of them who commit one or more of the following transgressions are not Muslims:

1. Believe that the Qur’an was altered

2. Believe that allthe Companions but a few apostatized

3. Say about the Mother of the Believers `A’ishah that which Allah hasdeclared her innocent of

4. Exaggerate the position oftheir imams (which were righteous descendants of theProphet) and elevate them to a position comparable to orabove that of the Prophets

Thus, the Prayer behind those who do one or more of these transgressions would be invalid. However,if one of them does not have an innovation that amounts todisbelief, then the Prayer behind him or her will be valid,though disliked because they still have rulings other than whatwe have and their form of Prayer is different from ours.Such differences are not like those among the Sunni schools of jurisprudence, which acknowledge the same sources but differon the interpretation.

Thus, you need to know the situation of the imam regarding that. If he has any of the abovementioned falsehoods, then you are not allowed to offer prayer behind him. However, in this case, you do not have to repeat the prayers you have performed behind him before, as stated by Sheikh `Abdurrahman Nasir As-Sa`dy, a prominent Muslim scholar, who said, 

If a Muslim came to know that he offered a prayer behind a non-Muslim while he did not know that he is non-Muslim, then according to the Hanbali school he or she would have to repeat the prayer. However, some scholars said that if the praying person does not know that the imam is kafir (non-Muslim), he or she does not have to repeat it. In my viewpoint, the latter opinion the sounder, and Almighty Allah knows best.

If he does not have any of the disbelieving innovation, then it is permissible to offer prayer behind him and you do not have to repeat the prayers you offered behind him since the prayer is valid.

As for the inquiry about why the Shiites curse `Aisha and dislike most of the companions including Abu Bakr, `Umar and `Uthman, there are many causes of that such as they wrongly believe that `Aisha abhorred `Ali and waged a war against him (battle of the Camel), and that these three companions along with those who supported them have wronged `Ali and seized caliphates which was Ali’s total right, according to their invalid views. There are many causes and they supported their claims with a great deal of false hadiths and reports.