Alphabet!

We learn the alphabet at the beginning of our study of English, and native speakers have to learn it too. Because most of us learned the alphabet a long time ago, it's hard to remember that it wasn't easy! For adults learning English, some of the sounds aren't easy to make or to recognize, depending on your native language or mother tongue. This expression is very good, I think, because the language you grow up with conditions your tongue as well as your mouth and what sounds you're able to make. For non-native speakers of Spanish and Catalan, for example, making the "rr" sound can be difficult. It's important to continue practicing the sounds of the alphabet, even though you know them, and especially if some of them, like "a" for example, are hard for you to recognize correctly when people say them. This can be a problem if you're on the phone and someone is spelling something, like his or her name.
For some good practice on pronunciation, I recommend New English File. Here's the link:
http://www.oup.com/elt/global/products/englishfile/preint/c_pronunciation/
This is the pre-intermediate one, and you could go to the other levels to practice.

P.S. This image is from a book by Peter Blake, Alphabets. Many thanks to www.dezeen.com