But what, But that
“Think no man so perfect but what he may err.” Say, “but that he may err.”
“I could not think but what he was insane.” Use but that.
But, If
“I should not wonder but the assembly would adjourn to-day.” Use if instead of but.
But, That
“I have no doubt but he will serve you well.” Say, “that he will serve you well.”
That, That
“I wished to show, by your own writings, that so far were you from being competent to teach others English composition, that you had need yourself to study its first principles.”—Moon, Dean’s English.
The second that is superfluous. This fault is very common with writers who use long sentences. The intervention of details between the first that and the clause which it is intended to introduce causes the writer to forget that he has used the introductory word, and prompts him to repeat it unconsciously.