that vs. if and whether
From my experience, about 95% of english speaking people (even educated people) employ this grammar (which I believe is incorrect, based on my school training in English, many moons ago, and which I hence detest and just cannot and will not adjust to !):
e.g.: “I wonder THAT this is correct”, rather than: “I wonder IF this is correct”, or:
“I wonder WHETHER this is correct”.
“I wonder THAT that is a fact”, rather than: “I wonder IF this is a fact” or:
“I wonder WHETHER OR NOT this is a fact”.
“I don’t know THAT it was cleaned much…” (from a radio personality this very evening)
IF or WHETHER must be used when there is uncertainty or doubt.
THAT should be used when there is certainty. E.g.: “I know that this is true.”