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/completely
partly/wholly
Therefore:
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
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.
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.
Let's look at the three sentences below.
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 ultimately this may become a question of style
and not definition or usage. Any of these three sentences could work just as
well with partiallyinstead 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
Source: http://www.learnersdictionary.com/qa/partially-and-partly
