Learn English – Indefinite Article: Why “a agreement” rather than “an agreement”

Google's n-gram viewer shows that both uses are possible. However, I would spell them differently:

1. The agreement was ratified by Parliament.
2. The agreement was ratified by the parliament.

In 1, "Parliament" is used as a proper noun and hence it doesn't admit the definite article "the". In 2, "parliament" is used as a common noun and hence the definite article "the" is necessary to indicate that the speaker expects us to know what parliament they are referring to.

In choosing a or an, the only thing that matters is the sound following the article, whether it's an adjective, a noun, or what: a for consonant sounds, and an for vowel sounds. There aren't different rules for different parts of speech.

One of the confusing things about English is that the same combinations of letters can sometimes be pronounced in different ways depending on the context. (By the way, my phonetic spellings are based on my Northeastern US accent, but the important parts regarding this issue should be the same everywhere.)

• unicorn: YOO-nih-corn
• unnatural: uhn-NATCH-er-al

And it happens that unisex is like unicorn, for the reasons that StoneyB mentions. So the appropriate article is a: a unisex _____ - because the y sound is considered a consonant sound.

Eu is another one that can be pronounced two ways:

• Europe: YER-up
• euphemism: YOO-fem-iz-em

but it's always a y sound when it's at the beginning of a word, so it's always a European vacation, a euphemistic explanation, and so forth.