Must is absolute; there is no choice. We must catch the next train (to arrive in time) Could is conditional; there is a choice. We could catch the next train (if there is no need to arrive before a given time). Could can also be used in place of 'would' for a polite request. Could you open the door please.

I must go = Je dois aller I could go = Je pouvais aller/Je pourrais aller As you can see, the meaning is very different!