The phrase “can’t hardly” is a colloquial expression that is sometimes heard in casual conversations. However, the phrase creates a double negative, a construction you usually want to avoid in writing. Negative words are often easy to spot. They can be “no” words, such as “not,” “nobody” and never” or negative adverbs like “hardly,” “barely” and “scarcely.” Keep your negatives limited—only one in a sentence or independent clause.
Once you identify a double negative, it’s usually easy to fix by dropping one of the negatives. Your sentence might read:
I can hardly wait for the game to start.
Or, it might read this way:
I can’t wait for the game to start.
Many rules have a caveat or two and this one is no exception. If you’re writing in the voice of a character who is prone to such usage in speech, then let the double negatives survive revision. (Show some restraint, though. Remember, less is often more when it comes to such quirks of voice.)
Also, in some instances, two negatives can capture a specific tone. You might use a word with a negative prefix, such as “unintelligent,” “inarticulate” or “nonsense,” with another negative word to state an idea positively but express reservation:
Finn is hardly unintelligent.
Consider the difference in tone when this sentence is stated in the positive:
Finn is intelligent.
Again, consider the voice of the character or narrator before using this approach.