What's the diffence between [be verb] VS. [be to verb]?
1-a. All you need to do is say sorry first.
1-b. All you need to do is to say sorry first.
2-a. Their top priority is save life.
2-b. Their top priority is to save life.
Sometimes I see sentences like above. But I really don't get what's the difference in nuace with or without 'to'. I find some of the native speakers say not to use 'to' in a certain context (I don't remember well), but how should I tell when to use 'to' or not?