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?

  • It's the presence of "do" in the subject noun phrase in 1-a that permits a bare infinitival, but it is optional, as can be seen in 1-b which is also OK. – BillJ Sep 29 '19 at 11:12

In 1 a and b there is no difference in nuance: they are completely free alternatives.

This is an unpredictable property of "do", and doesn't hold for other verbs. So, for example, *What I want is stop is not grammatical.

It also doesn't hold for other similar constructions, such as your second examples: your 2-b is grammatical, your 2-a is not.

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