When do i use a,an in the sentence?

hi,

Are someone can explain me ,when and how i use this form?

Thanks

Use "an" before a noun that starts with a vowel sound. Note that it's the way it sounds, not the letter that determines it.

Umbrella starts with a soft u, so it's an umbrella.

University starts with a hard u, so it's a university.

Pronunciation is the only determining factor.

If the noun or adjective being used starts with a VOWEL SOUND, then it should be preceded by "an."   Here are some examples:

An apple, an argument, an uncle, an old man, an amazing story, an autobiography.

If the noun or adjective being used starts with CONSONANT SOUND, then it should be preceded by "a."  Here are some examples:"

A book, a record, a candlestick, a unicorn (note that a hard "u" is pronounced as if it began with a "y" as YOO-ne-corn), a wonderful idea, a standing ovation.

We english native speakers still can't agree on historic.  (a historic vs. an historic)  I vote for "a historic event"