Learn English – the difference between “comprehensive” and “complete”

From the Oxford Corpus of English:

PER ANNUM

For 30 years, it had gone remorselessly ahead, at about 80% per annum.

An Elan bond, where the bondholder can exercise the right to be repaid in 18 months, currently yields 19 per cent per annum.

With this fresh impetus, the total edible oil processing capacity, including vanaspati, is expected to cross 20 million tonne per annum.

PER YEAR

We can currently get about 5% per year from investing in long-dated gilts, so we might aim to get 6% per year from the property.

Penetration of digital has hit 20 % in a year, with incremental revenue per subscriber at $22 per year and growing, he said.

This still leaves teachers $3,000 per year short of accountants, $17,000 short of computer systems analysts, and $25,000 short of engineers.

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.

Simply put, an expression is more like a saying, an idiomatic way of expressing something in less literal, less familiar terms:

How do you do? is just an expression: you don't expect someone to actually tell you how he is doing; he should just say how do you do? back, because it means little more than "hello".

A phrase is just any sequence of words, usually less than a sentence:

• rip them out
• even the Aztec women
• before the Spaniards
• reloading muskets
• because they could never have continued their rituals without capturing large numbers of enemy soldiers (this one is dubious, because "clause" would be a more specific and thus better name)

A great many expressions are also phrases, like put behind bars or heaven forbid.