I am learning about Hunt's (1965) system of parsing writing into T-units (minimal terminable units) and clauses. In this system, a T-unit is "one main clause with all the subordinate clauses attached to it [if any]... An and between two main clauses would always go with the second clause, beginning it just as coordinating conjunctions so often begin the sentences of mature writers"(p. 20), while a clause is "a structure with a subject and a finite verb (a verb with a tense marker). If subjects were coordinated, they merely lengthened the clause, and if any part of the verb phrase was coordinated, that also merely lengthened the clause" (p. 15). This system is useful for breaking up the writing of children, who often write long compound sentences connected by "and". Hunt provides this example of an essay written by a fourth grader:
I like the movie we saw about Moby Dick the white whale. The captain said if you can kill the white whale Moby Dick, I will give this gold to the one that can do it, and it is worth sixteen dollars. They tried and tried but while they were trying they killed a whale and used the oil for the lamps. They almost caught the white whale.
So, Hunt transcribes this into T-units (each on its own line), with clauses split by slashes:
(1) I like the movie / we saw about Moby Dick the white whale.
(2) The captain said / if you can kill the white whale Moby Dick / I will give this gold to the one / that can do it.
(3) And it is worth sixteen dollars.
(4) They tried and tried.
(5) But / while they were trying / they killed a whale and used the oil for the lamps.
(5) They almost caught the white whale.
I do not understand why there is a slash after "But" in T-unit 5--"but" does not contain its own noun-verb pair, and it seems to me that this "but" does not have a very different function than "and" (in T-unit 3). Is there some finer point of grammar that I'm missing?