You can't go back on your promise now!你现在不能食言

今天的几个句子是有关约定的：1.I'll try but I can't promise.我争取去，但不能保证。2.I'm sorry I can't help you. 对不起，我帮不了你。3.You can't tell anyone. 你不能告诉任何人。4.I'm not sure. 我没有把握。