I have recently been learning about complementisers and relative clauses etc. and how they relate to x-bar theory. It is a feature of English that some complementisers are optional, especially in spoken English, e.g. "I think (that) you are great".
This is also true for relative clauses: "The person (that) I saw yesterday"; "The ball (that is) on the table".
However, sometimes they are not optional: "The dog that is red" --> *"The dog red".
I suppose what I mean is, is there a way of telling when "that is" is a not-present optional complementiser, and when it is actually not present?
For example, with one of the above sentences, is "that is" really implied in the NP, "The ball on the table", or is it simply not there at all?
More particularly, with adjectives like 'available':
"The bread available in the bakery looks delicious".
Does this sentence contain an implied "that is", i.e. it is really a contracted way of saying "The bread THAT IS available in the bakery looks delicious", or does it actually not contain or imply at all "that is"?
This relates to x-bar theory, or rather phrase structure rules, in that normally adjective phrases are not allowed to come after a noun (except in poetic usage etc.), so if "available in the bakery" is an adjective phrase (not a relative clause), why can it come after the noun "bread"?
[in this question I have assumed that "that is" is a complementiser just like "that"; I admit I am not totally sure about this]