The Articles A-An
Exception: When using a word with "h" as "honorable" and "honest" one must make use of the particle "an" since the "h" is silent.
Example: A horse
When the word begins with a consonant the particle "a" should be used
Example: a glass water.
When the substantive begins with a vowel, the indefinite article "an" is used.
Example: An actor
Another exception:
"a" is used when "y" makes the same "h" sound.
Example: A unicorn