This is a run-on sentence:
It’s red and it has a broken seat but we found another seat to put on it.
At the very least, there should be a comma before "but", because a new subject is introduced.
It's red and it has a broken seat, but we found another seat to put on it.
The breakdown with this change would be:
Sentence Structure:
Subject 1 ("It", repeated twice) + compound predicate ("'s red and... ...has a broken seat")
Subject 2 ("we") + predicate ("found another seat to put on it")
Phrases:
"It" (subject 1), "it" (subject 1 repeated), "a broken seat", "we" (subject 2), "another seat", and "it" are noun phrases.
"‘s red", "has a broken seat", and "found another seat to put on it" are verb phrases.
"on it" is a prepositional phrase.
"red" is an adjective phrase.
"to put on it" is an infinitive phrase.
Clauses:
Two independent ("It is red and it has a broken seat" and "but we found another seat to put on it")
Stylistically, I would also change the first clause by removing the repeated subject "it." Another alternative would be to add a comma to produce three independent clauses.
It's red and has a broken seat, but we found another seat to put on it.
or
It's red, and it has a broken seat, but we found another seat to put on it.
The second alternative would require a change in analysis, since we now have three independent clauses, three subjects (the second "it" is a new subject because it is in a separate independent clause), and no compound predicate.