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Could you please help me with this?

1 The water is so cold it'll make your hands hurt./hurts my hands

2 We asked a professionals to reveiw the tape and they determined it was you who was on the phone and not someone else like you had said.

Thank you
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1 (b) . . . it'll hurt my hands / it hurts my hands

2 "We asked a professional" / "We asked professionals"
I'd suggest a comma after "else."
Comments  
Assuming in 1. you are talking about yourself, then it will "hurt my hands." There's no need to bring a "you" into the mix if you don't have to.
As for 2, since you say "a professional," don't use "they" later, which makes the sentence refer to more than one professional. Use "he said," or "she said," since you should know the gender of the professional, I'd hope.

As to the above answer, adding a comma after "else" changes the meaning. If you mean, that you said to me or to someone later that you had been on the phone, then use " they said that it was you who was on the phone and not someone else, like you said [to me]." But, if you mean that you told them that you had been on the phone, at that time that they questioned you, then don't add a comma. "...that you had been one the phone and not someone else as you said [to them then]."

These differences are subtle but need to be made. The comma adds a measure of uncertainty and time to the mix so that your saying this is separated from the events when it happened