L'articolo determinativo— The Definite Article

This is the grammatical name for the English word “the”.

1 Feminine nouns — easy.

If the feminine noun is singular, use LA (or abbreviate it to L'if the noun begins with a vowel). If the feminine noun is plural use LE and never abbreviate it, even if the noun begins with a vowel. For example:

2 Masculine nouns — tricky.

There is only one word for "the" for feminine nouns, but before a masculine noun you need to choose between IL and LO. You make the choice depending on how the masculine noun begins. You should find that you choose IL most of the time. Here are the rules:

1. Use IL and its plural I when the masculine noun begins with a consonant.
2. Use LO and its plural GLI when the masculine noun begins with a vowel, or z or “impure s” — i.e. the letter s followed by another consonant. LO can be abbreviated to L' before a word beginning with a vowel.
3. Remember that any nouns borrowed from another language are masculine and have no plural.

This table summarises all you need to know about masculine nouns and there articles:

Another explanation