When to say a and when to say an? The rule is very simple. Use a before singular nouns that begin with a consonant. Use an before singular nouns that begin with a vowel. But there are some exceptions, for example hour, unit.
Most plural nouns in English are regular. That means you can simply add -s or -es to make them plural. Easy! But there are some irregular nouns. They follow different rules or they do not follow any rules at all! You need to remember each of them :-)