When it comes to a clause, we'd naturally understand that each of them has a lexical class to it, and, the part of speech they serve as in a sentence.
Adjective + that clause has never been a rare usage. It's rather easy to get right the that clause in sentence 'It's important that this will be done' for example. The 'it' here is a formal subject, hence the that clause being the true subject. And the lexical class of this that clause is noun. Therefore, it is both a subject clause, and a noun clause.
But in the case of 'I'm sure + that clause', this gets me confused. 'I am sure' being a complete sentence which ends with a predicative, I don't know what the that clause after the predicative 'sure' serves to be in terms of part of speech. Sure, it should be a noun clause as always, but, what part of speech is it?
Thanks in advance!