Front
U2
A, an, the , no article (X)
Back
a = singular, countable noun: before a noun that starts with the sound of consonant (b,d,f,g,..)
an = singular, countable noun before a noun that starts with the sound of a vowel.
the = when it is clear who/ what you're talking about.
No article (X) = before plural (meervoud) countable nouns.