That

A conjunction.

yellowstarstruck I think that it is a ghost.

There are two clauses here: the main one, i.e. "I think" and subordinate "it is a ghost" connected by the conjunction "that".

yellowstarstruckIf you decide to use "that" what is it classed as grammatically? ... I think that it is a ghost.

The most recent term for it is "complementizer".

CJ

CalifJimThe most recent term for it is "complementizer".

I suppose that would make prepositions complementizers, too. If it ain't broken, why fix it?

yellowstarstruck Hi! If you decide to use "that" what is it classed grammatically? Thanks, Fulvio.1. I think it is a ghost.2. I think that it is a ghost.

It is also called "marginal subordinator".

I have given up answering questions about parts of speech. My answers would be out of date by the time I clicked on Post.

Mister MicawberI suppose that would make prepositions complementizers, too.

No. At least not since I've dabbled in the new terminology. Maybe they're complementizers too now, though I doubt it because they figure so often in adjuncts rather than in complements (in the morning; after the war; with a book). As far as I know there are just a few words t

CalifJimthere are just a few words that are considered complementizers. I believe that besides "that", "for" (It's not nice for him to say that) and "whether" (No one knows whether it's true) figure among them.

Well, I like that aggregation. It seems useful.

There is a hierarchy in labels: conjunction, subordinator, complementizer. Complementizer is narrower in meaning.