In Spanish, there are 4 different forms of the definite article ('the'). In order to know which form you need to use, you need to look at the noun and find out whether the noun is masculine, feminine, singular or plural. The definite article always matches the gender (masculine or feminine) and number (singular or plural) of the noun. Therefore, if the noun is masculine and plural, the article has to be masculine and plural.
Here are some examples:
Singular masculine:
Su hijo está en el colegio de lunes a viernes. His child is in the school from Monday to Friday.
Singular feminine:
Ayer compré la mesa que viste en mi habitación. Yesterday I bought the table that you saw in my room.
Plural masculine:
Los billetes de autobus son baratos. The bus tickets are cheap.
Plural feminine:
Tengo que ir al banco para recoger las tarjetas de crédito. I have to go to the bank to get the credit cards.
To know all the exceptions and take a quiz:
https://www.spanishdict.com/topics/show/5
Here are some examples:
Singular masculine:
Su hijo está en el colegio de lunes a viernes. His child is in the school from Monday to Friday.
Singular feminine:
Ayer compré la mesa que viste en mi habitación. Yesterday I bought the table that you saw in my room.
Plural masculine:
Los billetes de autobus son baratos. The bus tickets are cheap.
Plural feminine:
Tengo que ir al banco para recoger las tarjetas de crédito. I have to go to the bank to get the credit cards.
To know all the exceptions and take a quiz:
https://www.spanishdict.com/topics/show/5