确实是这样。
No doubt. *对别人的意见表示完全同意、赞成时。意为“毫无疑问”、“确实是这样”。
It's a hard job. (那是份很辛苦的工作。)
No doubt. (确实是这样。)
No doubt about it.
You said it. *比较随便的说法。
You can say that again. *比较随便的说法。
That's doubtful¹. (那是使人怀疑的。)

当然可以。
Fair enough. *因为fair“公平、公正的”是enough“足够、充分的”，所以意思是“很公平的”、“不得不同意对方的意见”。
I'll help you after dinner, okay? (晚饭以后我再帮你，行吗?)
Fair enough. (当然行!)
That's fair.
That's reasonable.

恐怕就是这样了。
I'm afraid so. *一种将不愉快的事情或对方不想听到的事情告诉对方并表示同情的语感。“很遗憾是这样”、“恐怕是这样”。
Did you lose your wallet? (你丢了钱包?)
I'm afraid so. (好像是。)
Do I have to do it again? (我还得重新干吗?)
I'm afraid so. (恐怕就是这样了。)

你的意见有道理。
Your opinion is reasonable. *reasonable “有道理的”、“合乎情理的”、“合理的”。
I don't think it's a good idea. (我觉得那不是个好主意。)
Your opinion is reasonable. (你的意见有道理。)
Your opinion stands to reason.
That's a reasonable opinion.
Your opinion is unreasonable². (你的意见没道理。)

如果你说是的话，那就是吧。
If you say so.
I'm telling the truth. (我在说事实。)
If you say so. (如果你说是的话，那就是吧。)

据我所知，是的。
Yeah, as far as I know.
Is he coming? (他来吗?)
Yeah, as far as I know. (来，据我所知。)
Yeah, as far as I can tell.
Yes, to the best of my knowledge. *恭敬的说法。
Yes, I think so. (对，我想是的。) 

我这就做。
I'm going to.
Have you finished your homework yet? (你已经做完作业了吧?)
I'm going to. (我这就做。)