What invalidates prayer, and does doubt invalidate prayer?

Prayer is invalidated by one of two matters: First, intentionally performing an act forbidden in prayer. Among these acts are: eating and drinking, speaking for reasons other than the prayer's benefit, excessive movement, and laughing. However, if any of these occur due to forgetfulness or ignorance, the prayer is not invalidated. And if the speech is for the benefit of the prayer, it is permissible, provided it is not excessive. As for the second matter that invalidates prayer, it is: neglecting what is obligatory within it. So, whoever intentionally neglects a pillar or an obligation, their prayer is invalidated. But if they neglect it due to forgetfulness, they should perform it (if it was a pillar) or perform the prostration of forgetfulness (if it was an obligation).