Перепишете каждое предложение дважды: в прошедшем и будущем времени. Заменяя модальные глаголы эквивалентами, где это необходимо:
1) I can not recite this poem
2) He can not join the party because he is busy
3) I can settle my own problems

1) I was not able to recite this poem.I'll not able to recite this poem.2) He was not able to join party because he was busy.He will not able to join party because he will be busy.3) I was able to settle my own problems.I'll able to settle my own problems.