Verb to be + ING / To

[Emanuelli]

Please, help me. I've been trying to understand the use of verb to be.
In fact, in some books, after verb to be, they use "to", in others, "ing". Example:

a) It's good to hear from you.
b) It was nice talking to you.
c) It's difficult to study English.
d) What you have to do is take the opportunity. (Why wasn't "to"used in this case???)

See, it's complicated. I really need your help.

Thanks.

[Alc112]

It was nice talking talking to you: Here you have to use To because the main verb needs it. You talk to someone

[Tdol]

In the examples you give a-c, you could use either a greund or the infinitive, though in most cases the infinitive is more common after an adjective. With d), you could use 'to', but it can be omitted to avoid repetition. I'm afraid this is an area where rules are defined by use rather than any grammatical function. With 'nice talking', I'd be more likely to use that for the past and the infinitive for the future, a pattern seen with verbs like 'regret'.