the word, 'mamoru' means 'keep', 'defend', or 'protect' and the parase '___ naito ikenai' means 'must' so, 'watashi wa yakusoku wo mamoranaito ikenai' = 'i have to keep my promise' 'watashi wa sono kurumaisu ni hito ga butsukaranai you ni mamoranaito ikenai' = 'i must protect the wheelchair not to be bumped by passersby'