Sentence adverbs are well established in English, but there are two – hopefully and thankfully – which have caused a lot of controversy. Learn more about hopefully as a sentence adverb. Here is a sentence in which thankfully is being used as a sentence adverb:
Thankfully, we didn’t have to wait long.
Many people are convinced that it’s wrong to use thankfully in this way. What’s the problem? It lies in the fact that you can't rewrite this type of sentence using the wording 'it is thankful that'. If you wanted to rewrite the previous sentence, you couldn’t say:
✗ It is thankful that we didn’t have to wait long.
You’d need to choose a different wording, for example:
As luck would have it, we didn’t have to wait long.
This leads people to the conclusion that thankfully should not be used as a sentence adverb. In fact, there are no very strong grammatical grounds for criticizing the use of thankfully as a sentence adverb: there aren't any rules that ban this sort of development of meaning. And there are other adverbs which behave in the same way but which haven’t attracted the same level of condemnation, e.g. frankly or briefly:
Frankly, I was pleased to leave. (i.e. to be frank, I was pleased to leave)
Briefly, the plot is as follows. (i.e. to be brief, the plot is as follows)
Nevertheless, you should be aware that some people strongly object to the use of thankfully as a sentence adverb. In view of this, it’s a good idea to be cautious about using them in formal writing such as job applications just in case your reader happens to be one of those people.
You can read more rules and guidelines about adverbs on the Oxford Dictionaries blog. Here you will find more examples of correct and incorrect use of adverbs.