Teaching "A" or "An" to ESL Students
If a noun begins with a vowel, then the article "an" is used. If it begins with a consonant, then "a" is used. This basic rule is fairly easy for ESL and EFL students to learn.
However, the more difficult point to teach is that we need to look at the pronunciation, not the spelling of the word. For example, "Lunch lasts for an hour." "Hour" begins with a consonant, but in American English, the "h" is silent, so the beginning sound is a vowel sound.
Another area students have problems with is in using initials or acronyms. For example, "Did you submit an RFP to the foundation?" Although "RFP" begins with a consonant, if I say the letter "R," it sounds like "ar". The beginning sound is a vowel sound.
I find that lots of exercises or drills are useful for reinforcing this point.
Also note, that I am assuming here that the difference between definite and indefinite articles has already been taught.