Q: I have a close friend and any number of acquaintances who say “I’m not for sure” when they’re not certain about something. I hold my tongue, but that “for” is like nails on a chalkboard to me. Is there anything remotely correct about it?
A: The prepositional phrase “for sure” is generally used as an adverb, not as an adjective.
So a sentence like “I don’t know for sure that it’s true” is idiomatically normal. (“For sure” functions as an adverb modifying the verb “know.”)
But a sentence like “I’m not for sure that it’s true” seems off-kilter. (“For sure” here functions as an adjective.)
Obviously, “for” is redundant in the adjectival usage. In sentences like “I’m sure” and “I’m not sure,” the adjective “sure” is sufficient. It follows the verb “be” and thus it essentially modifies the subject.
Yet many people add “for,” as in “I’m for sure we’re going to be late” … “I’m not for sure where I left my keys.”
The negative version is especially common. In a Google search, “I’m for sure” got 1.6 million hits while “I’m not for sure” turned up 4.7 million.
Why does “for” creep into the picture? We suspect this trend was influenced by other common uses of “for sure.”
These include the colloquial expression “that’s for sure,” generally used alone or placed at the end of a sentence. This is another adjectival usage.
But in the other familiar usages, “for sure” functions as an adverb, since it modifies a verb. Examples: “She’s going for sure” … “He can’t say for sure” … “They’ll know for sure” … “Tell them for sure that you’re quitting.” Such usages often intensify or add emphasis to a statement.
The American Heritage Dictionary of the English Language (5th ed.) labels this adverbial use of “for sure” informal and defines it as meaning “certainly” or “unquestionably.”
The Oxford English Dictionary labels it colloquial and says it means “as or for a certainty, undoubtedly.”
In its entry for the preposition “for,” the OED says it’s sometimes used adverbially before an adjective to create a sense of “attributed or assumed character.”
This kind of construction is evident in many familiar verb phrases—not only “to know for sure” and “to say for certain,” but also “to take for granted,” “to leave for dead,” “to give it up for lost,” and “to take … for better or for worse,” from the marriage vow in the Book of Common Prayer (1549).
In short, “for sure” often functions as an adverb and nobody finds that remarkable. But it’s less idiomatic when used as an adjective (as in “She’s for sure that it’s over”).
So far, we haven’t found any evidence that lexicographers or usage authorities have commented on the adjectival usage that bugs you.
What’s the conclusion? Well, it’s hard to ignore millions of Google hits, but we find “I’m for sure” and “I’m not for sure” to be unidiomatic. The “for” isn’t necessary and adds nothing. That’s for sure.
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