Is “whom” dying out?
In casual speech, “who” is now used for both roles (“Who did you invite?”), and dictionaries such as Merriam-Webster accept it informally. In formal writing, after a preposition (“to whom”, “with whom”) and in set phrases, “whom” is still expected.
The “do you think” trap
Phrases like “do you think”, “I believe” or “they said” can hide the real structure. Remove them first: “Who (do you think) will win?” becomes “Who will win?”, so it’s “who”.
Catch it automatically
The grammar checker flags common mix-ups like this one by looking at the words around them, and the sentence checker can explain any sentence you’re unsure about.