Specific 'it/they', etc. and non-specific 'one/some', etc

Obligatory subjects: 'it', 'they', 'one', 'some' (for things)

It and they are used as subjects if the reference is specific: specific: Did the letter I've been expecting come?

- Yes, it came this morning (the + singular noun = it)
Did the letters I've been expecting come?

- Yes, they came this morning, (the + plural noun = they)
One and some, functioning on their own as pronouns, can be used as
subjects if the reference is non-specific:
non-specific: Did a letter come for me?

- Yes, one came/some came for you this morning
(a/an + singular noun = one)

Did any letters come for me⁹

- Yes, some came/one came for you this morning
(any/some + plural noun = some in a positive answer or
none in a negative answer)

Obligatory objects: 'it', 'them', 'one', 'some', 'any' (for things)

An object is obligatory after transitive verbs, such as enjoy or make, and verbs which are being used transitively, such as play [> App 1]. It, them or a noun must be used as objects when the reference is specific [> 4.16.1]:

What do you think of this cake?

- I like it/I don't like it (Not */ like/don't like*)
What do you think of these cakes?

- I like them/I don't like them (Not */ like/don't like*)
One must be used as an object when it stands for a/an + countable
noun (i.e. the reference is non-specific) [> 4.16.1]:

Have a biscuit - I've had one/I don't want one thank you Would you like a drink? - I'd love one thank you

Some and any [> 5 10] must be used as objects when there is a non-specific reference to uncountable nouns and plural countables:

Have you got any sugar? Can you lend me some please⁹ Sorry, I haven't got any (to spare).

Have you got any drawing-pins? Can I borrow some please⁹

- I'm afraid I haven't got any (to spare)