We know that omitting to do some things can be sinful. But which omissions are so grave that they may become mortal sins if all the conditions are met?
Also, if neglecting to do the corporal works of mercy is grave, for example, then this expands the scope of inquiry: how often should I do them? If it is grave not to give to the poor, how much should be given? Church teaching is very unspecific on these positive duties so I can’t say for sure whether my omissions are grave.