I was perusing the net on the hunt for reliable sources to help me understand the subjunctive mood when I came across this section and it reminded me of this question!
I'll quote it in almost its entirety because first, it's very clear and it might be of help to other users and secondly, I liked it.
I. A phrase is a collection of words that may have nouns or verbals,
but it does not have a subject doing a verb. The following are
examples of phrases:
- leaving behind the dog
- smashing into a fence
- before the first test
- after the devastation
- between ignorance and intelligence
- broken into thousands of pieces
- because of her glittering smile
In these examples above, you will find nouns (dog, fence, test, devastation, ignorance,
intelligence, thousands, pieces). You also have some verbals (leaving,
smashing), but in no case is the noun functioning as a subject doing a
predicate verb. They are all phrases.
II. A clause is a collection of words that has a subject that is
actively doing a verb. The following are examples of clauses:
- since she laughs at diffident men
- I despise individuals of low character
- when the saints go marching in
- Obediah Simpson is uglier than a rabid raccoon
- because she smiled at him.
In the examples above, we find either a noun or a pronoun that is a subject (bold) attached to a predicate verb (italics) in each case:
- since she laughs at diffident men
- I despise individuals of low character
- when the saints go marching in
- Obediah Simpson is uglier than a rabid raccoon
- because she smiled at him
III. If the clause could stand by itself, and form a complete sentence with punctuation, we call the clause an independent clause. The following are independent clauses:
- I despise individuals of low character
- Obediah Simpson is uglier than a rabid racoon
We could easily turn independent clauses into complete sentences by adding appropriate punctuation marks. We might say, "I despise individuals of low character." Or we might write, "Obediah Simpson is uglier than a rabid racoon!" We call them independent because these types of clauses can stand independently by themselves, without any extra words attached, and be complete sentences.
Best Answer
From the Oxford Corpus of English:
PER ANNUM
PER YEAR
These example sentences are representative of the most common uses of these two phrases and, as one can see, there is no real difference between per annum and per year in usage.
As kiamlaluno says, per annum is traditionally used more in financial contexts than per year, but these sentences show that per year is also perfectly acceptable.