PLURALS OF NOUNS
As explained on Nouns 02, most nouns in French form their plurals by adding "s". However a few that end in '-s' or '-z' don't change at all - like "le fils" (son), "le bras" (arm), "le gaz" (gas), "le nez" (nose), "l'autobus (m)" (bus). There's also an '-x' word "la noix" which stays the same in the plural - "les noix".
Some words in French add an "x" instead of an "s", in the plural (see those on Nouns 02, which are fairly simple). However there are a few 'odd' plurals for words ending in '-l', where the plurals end in '-x'.
There are (again) 'exceptions' like "le festival" (festival) and "les festivals", "le bal" (ball) and "les bals", "le final" (final) and "les finals", but the others you'll need to learn.
You may also wish to make a note of "les cheveux (m)" (hair) - always plural, and "le pantalon" (trousers), "le collant" (tights/leggings), "le pyjama" (pyjamas), "la salopette" (dungarees), "la blouse" (overalls), and "le jean" (jeans), all translated by the singular.