Indefinite Article

Form
A and and are form of the indefinite article. A is used before a consonant sound and an is used before a vowel sound. (A/an ជាទំរង់របស់ការន្តមិនជាក់លាក់)

Use

 * It is used before a singular countable noun. When it is mentioned for the first time. (នៅពេលយើងលើកយកមកនិយាយលើកដំបូង), They live in a flat
 * When it is one example of a class. (ជាពាក្យគំរូ), e.g. A dog needs exercise
 * As a complement. (ជាពាក្យបំពេញ), e.g. She is a doctor.
 * In exclamations. (ក្នុងទំរង់ឧទាន)​, e.g. What a lovely day!
 * Means "per" (ក្នុងមួយ), e.g. 5 USD a kilo
 * In certain phrases of quantity (ឈ្លាបញ្ជាក់ពីចំនួន), a lot of
 * Before whole number and fraction (មុខចំនួនគត់ និង ប្រភាគ), a hundred.

Omission

 * It isn't used before a plural noun (មិនប្រើពីមុខ នាមពហុវចនៈ)
 * Mass noun (នាមមិនអាចរាប់បាន)
 * Name of meals (ឈ្មោះអាហារ)

Note:
I have breakfast.

I have a big breakfast.

I have a breakfast with the king.

a/an and one

 * We use both in counting and measuring (ក្នុងរង្វាស់យើងប្រើទាំង២), a/one pound
 * One is clearer than a/an (One ច្បាស់ជាង a/an)
 * One is used with another/(the) others and day/week/...
 * One = a/an + noun (one មានន័យស្មើនឹង a/an + noun)