Q: What are your thoughts about using “I bet” versus “I’ll bet” to introduce a statement? I prefer “I’ll bet,” but I can’t explain why.
A: The verb “bet” has several meanings in addition to its usual gambling sense:
1. to agree (“I was bummed out” … “I bet you were”); 2. to disagree (“I’ll really stick to my diet this time” … “Yeah, I bet”); 3. to mean certainly (“You bet I’ll be there”); 4. to say you’re fairly sure (“I bet she felt crummy,” “I bet he’ll forget,” “I’ll bet you come late tomorrow,” “I’ll bet they’re late again”).
In #4, the usage you’re asking about, “I bet” or “I’ll bet” introduces a subordinate construction. You can find examples in standard dictionaries for both “I bet” and “I’ll bet” used in this sense, though “I bet” is more common.
In our opinion, “I bet” (present tense) sounds more natural when the subordinate construction is in the past tense (“I bet she felt crummy”) or the future tense (“I bet he’ll forget”).
But “I’ll bet” (future tense) seems more idiomatic when the complement uses the present tense to express the future, either with a time element (“I’ll bet you come late tomorrow”) or without a time element (“I’ll bet they’re late again”).
A few years ago, we wrote a post about the futurate—a usage that expresses the future with a tense not normally used for it, as in “He arrives Saturday.”
Something similar is at work when we use “bet,” “wager,” and “hope” to talk about the future. These verbs, according to The Cambridge Grammar of the English Language, often introduce “subordinate constructions allowing pragmatically unrestricted futures.”
For example, the underlined complements of “bet” in “I’ll bet you come late tomorrow” and “I’ll bet they’re late again” express what the Cambridge Grammar calls a “deictic future time.” The sense of a deictic expression depends on how it’s used.
As the authors, Rodney Huddleston and Geoffrey K. Pullum, explain, “The construction generalises to the deverbal nouns bet, wager, hope.” In other words, “I’ll bet they’re late again” is another way of saying “My bet is that they’re late again.”
Getting back to your question, we’ve generally (though not always) used “I’ll bet” or “we’ll bet” with complements that indicate the future but aren’t expressed in the future tense.
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