Bao, a Chinese teacher of English, asked a question about the difference between partially and partly. This is a question that native speakers of English also sometimes ask. The distinction is very subtle because the definitions of the two words are nearly identical.
Let me say first that these two words can nearly always be used interchangeably. We will see that there may be patterns of use that favor one word over the other, but that does not mean that one word is always correct and the other incorrect in any given sentence. Also, to the native ear, partially sounds a bit more formal than partly, and sometimes the choice of partially is due to a formal or official context or a desire for a more elevated tone that has nothing to do with the meaning of the word itself. There are indeed differences in usage between the adjectives partial and part, but those differences are very hard to perceive when discussing their related adverbs.
The author of a famous book on English usage, H.W. Fowler, suggested making the distinction between the two words more clear by contrasting opposing terms to partially and partly in order to orient them:
partially = incompletely = to a limited degree
He has only partially succeeded in his mission.
partly = in part = as regards a part and not the whole
My solution came to me partly from experience and partly from instinct.
Either partially or partly could plausibly be used in both of the above example sentences, but they illustrate Fowler's suggestion that a distinction might be made in such cases. The problem with Fowler's rule is that there is plenty of evidence of overlap in the use of these words in the history of English. Fowler made his suggested rule in 1926, and while it might be a helpful distinction, it's impossible to change the course of language evolution simply by declaring a rule.
However, even though the the words are often used interchangeably, there is some differentiation that we can see by observing usage patterns.
Partially is used more often than partly to modify an adjective or past participle that names or suggests a process:
His face was partially concealed by a beard.
The snow had partially melted.
Our vacation was partially paid for by the company.
Partly is used more often than partially before clauses and phrases offered as explanation:
We trusted him partly because he was elderly.
Partly for this reason, we decided not to buy the house.
I called him again, partly to reassure him.
It must be said that there are plenty of exceptions to this general trend, and only time will tell whether the process of differentiation will continue. None of the above examples would sound incorrect to a native speaker if the words partially and partly were exchanged.
Finally, let's look at three sentences Bao submitted. All use partly, but can any of them also use partially?
1. I didn't enjoy the trip very much, partly because of the weather.
2. This he did with difficulty, partly on account of his bad eyesight.
3. It was partly their responsibilty.
We can try to apply Fowler's rule or look at the usage trends, but to my mind ultimately this becomes a question of style and not definition or usage. I would say that any of these three sentences could work just as well with partially instead of partly. The learner of English should focus more attention on those few cases that show nearly consistent use of one or the other:
partly cloudy/sunny skies
partially hydrogenated oils
Can anyone think of others? Learning a handful of these idiomatic uses will be more useful than trying to apply very subtle rules and guidelines.