I think that…*最简单的
It seems to me that…*可以用作表达意见，或者summarize或者confirm对方说的话。
I would argue that…*这里面用would来弱化语气。
Just to go along with what you said…*这句话真的很好用，承上启下。这句话还好在，它使你现在插话看上去legitimate,因为你是为了和前面的那个人go along啊。不赶紧接话，不就along不上了嘛。
Going back to what xxx said…* 同上。不过一般是你已经说了几句，此事，想要relate to something 别人刚刚提到过或者蜻蜓点水说道的东西。这句话的好处就是，一能bring others on board说看我的说法和你是有一致之处的，二能表现他刚才说的东西你还有印象。
I agree with you.*最简单的
Building on what XX has just said …
I am with you…，but [main point]. *这里I am with you不一定表示同意也不一定表示反对，它主要的意思是说"你说的我听懂了/我跟上你说的了。"
I follow your logic, but have you thought about/considered this…[main point]…？*和上面一句话的逻辑
I see what you are saying, but I’m not sure I would argue that…*和上面两句话的逻辑一样。
With all due respect, I think…
Correct me if I'm wrong, but isn’t [a different topic]…？
I'm not an expert on…，it seems to me that…
I might be wrong, but…
I suspect that I lack some [background] information about this aspect, but here is my two cents.
I am not sure about this…，you might want to check with Professor X…Would you mind letting me know what he/she says?
[别人说了一堆之后]This is one way to look at it. Another way to look at it might be…
Just to clarify…
May I ask a follow-up question?
Could you say more（about it）？*不确定自己是不是听懂了，请别人多解释一些。
Could you elaborate on this?
That's an amazing model/useful concept, but it occurred to me that [this/something] might also play a role…Have you considered …？
Thanks for your talk. I completely agree with your conclusion, but as I'm sure you’re aware, Dr. XX explains this in a different way, and I was wondering if you could address her theory. * "but as I'm sure you’re aware"完全是给对方一个台阶下，可能那个理论他根本没有考虑过。但是如果对方是很senior的，我们也需要客套一下。
I was wondering if I could talk to you about…*一般都是用过去时。虽然was wondering/hoping这种进行时看起来很怪，但是用法就是这样子的。
I was hoping that if I could talk to you about…
I was wondering if it'd be possible that you…*
Sorry to bother you, Professor X…
I realize this is an unusual request…
I realize that you are very busy…
Thanks for your time.
Thanks in advance for your help.
It would be great if you could…*这个是最常用的，尤其是email里面。