1. 介词可以接名词性wh-从句（不能接that从句），也可以接wh-+不定式。

I'm unsure about what to do next. = I'm unsure about what I should do next.

2. agree的及物性主要与其语义相关，也与英美差异有关。

1）agree表示“同意”

1.1）主要作不及物动词，接宾语时需加介词 ≈approve of

I totally agree with you! ==> I totally agree with what you have said!

1.2）作及物动词，只接that从句，that可省略

2）agree表示“批准”，为及物动词 ≈approve

The company agreed a deal worth $100 million.

3）agree表示“商定，达成一致” ≈decide/decide on，美式英语视为不及物动词，英式英语视为及物动词，总体上加不加on都可以。

I'm sure we can all agree on what to do next.

We couldn't agree what to do.

agree on a date

agree a price

以上是一些不成熟也不全面的小结，仅供参考。

基于以上粗浅的小结，网友问题中的句3表示同意别人所说的话，所以必须加with，句2、4、5表示商定，加不加on都可以。

1... agree that... （ that 引导宾语从句）

2 we couldn't agree what to do （what to do一起作宾语）

这里agree是及物动词

3 I don't agree with what you said （what you said作agree with的宾语）

4 we don't agree on what we ought to do （what we ought to do作agree on的宾语）

5 we don't agree on where to go （where to go 作agree on的宾语）

这里agree是不及物动词

介词后能直接接从句吗?【答】能够。

2345有什么区别呀？【答】表示同意某人或者某人的意见、想法、分析、解释等用 agree with ；表示双方通过协商而取得一致意见或者达成协议，用agree on。

【一】agree当作及物动词，可以接以下几种形式

【1】agree +名词(sth)

We must agree the arrangements for this.我们必须同意为此所做的安排。

I agree your draft.我同意你的草案。

【2】agree+ that从句

I agreed that it was a mistake.我承认这是一个错误。

I agree that your plan is better.我承认你的计划更好些。

He agreed that she had improved a great deal.他承认她比以前进步多了。

【3】agree +wh-从句

We could not agree how it should be done.
我们就这事应该如何做未能达成协议。

They could not agree who should take the lead.他们还没商定好谁来领头。

【4】agree +不定式to do

Mr. Brown has agreed to speak at the meeting.
布朗先生同意在会上讲话。

The unemployed workers agreed to take further action.
失业工人一致同意采取进一步的行动。

We agreed to accept the offer.我们同意接受援助。

【5】agree +疑问词+to do

We agreed how to try it.
我们就怎样试行取得了一致意见。

We could not agree how best to do it.
如何将此事做得最好,我们未能取得一致意见。

We could not agree when to meet.
我们在什么时候碰头这件事情上不能取得一致意见。

【二】agree当作不及物动词，后面可以与介词/about/to/on/upon/with构成搭配，可以理解介词短语作状语。

【1】agree about+名词/代词 sth

Both the parties agreed about that.双方对此意见一致。

They never agree about politics.他们在政治观点上从不一致。

Have you agreed about the price yet?你们就价格问题达成协议了吗?

【2】agree on/upon sth

They agreed on a date for the next meeting.他们就下次会议的日期达成了协议。

They agreed on a cease fire.他们就停火问题达成协议。

【3】agree to sth

Do you agree to the plan?你同意这个计划吗?

I told him about my plan and he at once agreed to it.

我把计划告诉他,他马上就同意了。

We agreed to their proposal.我们同意了他们的建议。

【4】agree with sb/sth

I agree entirely with you.我完全同意你的看法。

In a way I agree with Mary.在某些方面我同意玛丽的意见。

In the main,I agree with Edward.我基本上同意爱德华的意见

I agreed with your estimate of the situation.我同意你对形势的估计。

Do you agree with what he said?他说的你同意吗?

I quite agree with what you say.我完全同意你所说的。

【5】agree with sb about sth

I agree with him about the way of solving the problem.我同意他的解决问题的方法。

She agrees with him about this matter.她与他在这一问题上意见一致。

备注(1):agree with后可接意见、看法、观点。

备注(2):agree to后可接建议、计划、安排。