In short: "Agree with" -person- "Agree to" -action (verb) "Agree on" -time, idea- For example: She *agreed with Sarah* that they should watch a movie. They *agreed to go* to the movies at 9 PM. We should *agree on when* to meet beforehand. Does that help?

I agree with the people above, but there are some things, like "plans," where all 3 prepositions are OK, but have different meanings. I agreed with that plan= I thought that that plan was a good plan with correct ideas. I agreed to that plan= I agreed to follow that plan=I said, "I will follow and comply with that plan" Bob and I agreed on that plan= the plan was the final result of our negotiations. "My company was trying to decide between Plan A and Plan B. I agreed with plan A: I thought it was a much better idea. However, my boss and the board of directors agreed on plan B, so now I have to agree to follow plan B, even though I think it's stupid."

"I agree with you", "I do not agree to sign this document", "Let's not agree on the fine details right now".