When is "i", "gli" and "le" used? First I thought that le and gli was used only when the noun began with a vowel because it seemed to be the pattern, but then le birre appears. Why?
See here: http://italian.about.com/library/fare/blfare110a.htm
"i" and "gli" is used for plural of masculine noun. "le" and "l + apostrophe" is for plural of feminine noun. And yes, birra (beer) is feminine in Italian. See previous comment for further info.
my mistake, feminine plural does have only "le" before noun, not "l + apostrophe"