How do you change nouns from singular to plural?

To start with, we're going to need to change the original 'le/la/un/une' (the article) to 'les' or 'des', and these apply whether the noun is masculine or feminine - we'd use 'les' if we want to say 'the', and 'des' if we want to say 'some'. So, for example, les garçons (the boys) or les filles (the girls) vs. des garçons (some boys) or des filles (some girls). Normally, we only need to add an -s onto a singular noun to show that it's plural. So, for example, un chien (a dog) becomes des chiens (dogs) or une tortue (a tortoise) becomes des tortues (tortoises). But, there are some nouns that work a bit differently (irregular nouns). If a noun ends with -eu in its singular form, it changes to have an ending of -eux in its plural form; if it ends with -eau it changes to -eaux; if it ends with -ou it changes to -oux. So, un feu (a fire) changes to des feux (fires); un gâteau (a cake) changes to des gâteaux (cakes); un bijou (a jewel) changes to des bijoux (jewels). Also, another exception is that nouns ending in -s, -x or -z in their singular form don't change in the plural. So, looking to 'un cas' (a case), this will simply become 'des cas': the only part that changes is the 'un', and 'cas' stays exactly the same. In the same way, un choix (a choice) goes to des choix, and un gaz (a gas) goes to des gaz.