Recently, when talking with a student, I was asked what the difference was between "He clearly did it" and "Clearly, he did it". I have to admit that I found it difficult to give a good answer, because I'd never had to explain it before. There definitely is a difference; as a native speaker I'd sometimes use one, and sometimes the other, but why?

So I did what any good teacher would do: I told the student I'd have to look it up. It's okay to not know everything. Now that I've looked it up, this is how I would explain the difference.

In the first sentence, "clearly" is a normal adverb. It modifies the verb "did" and answers the question, how / in what manner?

He did it. How did he do it? He clearly did it [i.e. it's easy to see that he did it]

In the second case, clearly acts on the whole sentence. It expresses the speaker's feelings about it. When adverbs are used this way they're called "sentence adverbs". Sentence adverbs are similar to interjections in how they reflect the speaker's attitude.

Imagine this situation: you and I are trying to solve a mystery. Today is Karen's birthday. We bought her a cake, but someone ate the entire thing! We decide to look at the social media accounts of all the suspects, and one of them, Tom, just posted a picture of himself with frosting all over his face and the hashtag #CakeBandit. When you see this you might say:

Clearly, he did it.

Here, clearly means "it's obviously true; anyone who saw this picture would agree". Or you might say,

Wow! He did it.

The interjection wow shows that you're suprised --maybe before you saw the picture you thought someone else ate the cake.

To repeat: sentence adverbs, like interjections, show the speaker's feelings about the sentence. Like interjections, they're not necessary, but people use them all the time. Here are several commonly used sentence adverbs:

Obviously means almost the same thing as clearly: that anyone can see something is true.

Obviously, we have to buy another cake.

Frankly means "what I am about to say it true".

Frankly, I never trusted him.

Hopefully means "I hope what I am about to say will happen".

Hopefully, he can get treatment for his cake addiction.

Other sentence adverbs include plainly, honestly, luckily and surprisingly. If you are not sure how to use them, remember: Always Check The Dictionary!