1. agree with
(1) 表示同意某人或某人的意见、想法、分析、解释等 (即持同一观点)。如:
I quite agree with you. 我完全同意你的意见。
We agree with what you say. 我们同意你说的。
(2) 表示“(食物、天气、工作等)对……适宜”。如:
The food does not agree with me. 这食物对我不适合。
Hard work does not agree with him. 艰苦的工作对他不适宜。
(3) 表示“与……一致”。如:
His story agrees with the facts. 他的陈述与事实相符。
A verb must agree with its subject in person and number. 动词必须和它的主语在人称和数方面保持一致。

2. agree to
(1) 后接某些名词,表示同意或接受某事,尤其指别人提出的某事,有时可能是自己不喜欢的事。
He agree to the plan (the date). 他同意了这个计划(日期)。
We agreed to their arrangement. 我们同意了他们的安排。
I was forced to agree to it, but at heart I didn’t quite agree with it. 我被迫答应,但内心并不完全同意。
(2) 后接动词原形(此时to是不定式符号)或动名词(一般有逻辑主语,此时 to 是介词)。如:
We agreed to leave early. 我们同意早点出发。
She agreed to my going home. 她同意我回去。
注:英语不说 agree sb to do sth。如不说:
* She agreed me to go home.

3. agree on
(1) 主要指双方通过协商而取得一致意见或达成协议。如:
Can we agree on a price (a date)? 我们能不能商定一个价格(日期)?
Both sides agreed on these terms. 双方都同意这些条件。
注:在正式文体中,有时可省略介词 on。如:
Can we agree a price (date)?
(2) 后接动名词,表示同意做某事。如:
He agreed on helping us. 他同意帮助我们。
注:与 agree to do sth 大致同义。所以上句也可说成:
He agreed to help us.