a pie tastes good x a tastes good
the pies taste good x the taste good
x it pie tastes good it tastes good
x they pie taste good they taste good
English has two tricky little groups of words. One group – including a and the – is usually used in front of a noun to make a basic noun phrase. The other group – including it and they – is usually used to replace a noun phrase.
But there are other groups of words which can be used both ways:
this pie tastes good this tastes good
those pie taste good those taste good
The other main members of this group are that and these.
The apostrophe-s or s-apostrophe constructions can also be used both ways:
Nanna’s pie tastes good Nanna’s tastes good
my grandmother’s pies taste good my grandmother’s taste good
Finally, there is a group of words which are obviously related, but one of each pair is used in front of a noun and the other is used by itself:
her pie tastes good x her tastes good
x hers pies taste good hers taste good
The other main members of this group are my-mine, your-yours, our-ours and their-theirs. There are two exceptions. For his, the dependent and independent forms are identical:
his pie tastes good his tastes good
and its does not occur independently:
its pies taste good x its taste good
Note that the independent forms in the right-hand column can only be used in contexts in which the item/s under discussion is/are known to the speaker/writer and the listener/reader. I can’t just post on Facebook ‘This tastes good’. If I do, you are entitled to ask ‘What is this?’. I can, however, post ‘This pie tastes good’, in which case context it useful, but not required; you can ask ‘Where are you?’ or ‘What kind of pie is it?’, but you can’t really ask ‘What is pie?’. I can say ‘This tastes good’ if we are eating together, but if I say it while we are talking on the phone, you are likewise entitled to ask ‘What is this?’.
Almost every native speaker masters this by the age of three. Many second language learners struggle with this for a long, long time.
(There is technical terminology for all of these, which I decided to skip. I don’t want to confuse me or you.)