In the sentences you've provided, "who" and "whom" are used in contexts where "whom" would be correct if it's functioning as the object of a verb or a preposition.
Let's analyze the sentences:
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Who wants to help clean the pool so that it will be ready when summer begins?
- This sentence is correct as "who" is the subject of the verb "wants."
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The water from the faucet has turned brown, but I'm not sure who to call about it.
- "who" is the subject of the infinitive "to call," so this is correct.
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Bryan, who loves playing soccer, was disappointed when he was cut from the team.
- "who" refers to "Bryan" as the subject of the clause "who loves playing soccer," so this is correct.
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Dorothy is a professional pastry chef who can bake a great Chantilly tart.
- "who" is the subject of the clause "who can bake," so this is correct.
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Who do you think should be the next president of the science club?
- "Who" is the subject of the verb "do think," so this is correct.
None of the sentences you've provided need "who" to be replaced with "whom."
However, if one was to make this change, the proper use of "whom" can be demonstrated as follows:
- In an alternate structure, such as if it were rephrased: "To whom do you think I should speak?"
But of the sentences you provided, they all use "who" correctly according to standard grammar rules.