There is a 100 percent chance that this sentence is partially incorrect. There is zero chance, however, that this sentence is partly incorrect.
Why? The answer, as any even partially observant reader might notice, lies in each sentence’s final adverb.
Admittedly, the distinction between “partially” and “partly” is of interest only to the greatest of pedants. Thus, one should not be surprised to learn that I find the difference significant and necessary.
“Partially” is to be used when one is describing part of something without a “measurable whole”: He partially discovered the meaning of life yesterday.
“Partly” should be used when a measurable whole exists: She partly finished her glass of water.
There’s a simple test for this: If you can identify a tangible remaining part, use “partly.” In the first example, notice that one cannot exactly measure the part of the meaning of life he (whomever he may be) has not discovered. In the second example, the fraction of the water she didn’t consume is clearly observable.
Take heed, and you’ll never again be only partially sure that your sentences are partly correct.