Which word best fills in the blank?
If they ______ that the weather was going to be so bad, they wouldn't have gone to the beach.

Using "know" in this context is incorrect because it is in the present tense. The sentence is discussing a past situation, where the correct tense should reflect their previous knowledge about the weather, making "know" unsuitable for this construction.

The option "knew" is in the simple past tense, which does not convey the conditional aspect implied by the sentence. While it suggests that the subject had knowledge in the past, it fails to establish the necessary relationship between their knowledge and their choice to go to the beach, which is better expressed with the past perfect tense.

The phrase "were knowing" is grammatically incorrect and awkward in this context. The continuous form is not appropriate here, as the sentence requires a completed action in the past concerning their decision, which makes "were knowing" an improper choice.