Q: If a person can stand but cannot make sajdah can he sit and make signs?
A:Yes, he can sit and make signs for ruku and sajdah. In fact it is better for him to sit and make signs than stand and make signs. When making signs for ruku and sajdah he should compulsorily make his sajdah lower than his ruku if he is able. If, despite being able to bend lower for sajdah, one does not do so then the sajdah will not be valid thus rendering the entire salaah invalid. (Durrul Mukhtaar)