Hi! 1 is correct. 2 and 3 are wrong. Am I right? Thanks, Fulvio
1. John knows Mary likes spicy food.
2. John knows whether Mary likes spicy food.
3. John knows if Mary likes spicy food.
All those sentences are possible. Formally, (2) is superior to (3), in my opinion, but in everyday English (3) would be normal. (1) implies that Mary does like spicy food.
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All those sentences are possible. Formally, (2) is superior to (3), in my opinion, but in everyday English (3) would be normal.
(1) implies that Mary does like spicy food. In careful written English, I prefer "John knows that Mary likes spicy food."
(2) and (3) do not specify whether or not she likes spicy food, just that John knows which it is.