Note the word endings, and how they change in their plural forms:

Words ending in "a" or "e" get an "s". casa-cases, note that the ending "a" always changes to "e" before the "s"

Words ending in "à" - the ending becomes "ans" humà-humans

Words ending in "è" or "é" - the ending becomes "ens" cinquè-cinquens

Words ending in "í" - the ending becomes "ins" camí-camins

Words ending in "ò" or "ó" - the ending becomes "ons" camió-camions

Words ending in "ú" - the ending becomes "uns" oportú-opportuns

Words ending in "s" - the ending becomes "ssos" gos-gossos

Words ending in "ix" - the ending becomes "ixos" baix-baixos

Words ending in "tx" - the ending becomes "txos" esquitx-esquitxos

Words ending in "ig" - the ending becomes "itjos" desig-desitjos

Words ending in "aig" - the ending becomes "ajos" assaig-assajos

And finally, words ending in any other consonant - simply get an extra "s" curt-curts

However, be careful with the following orthographic changes.

-ca becomes -ques vaca-vaques

-ga becomes -gues euga-eugues

-qua becomes -qües Pasqua-Pasqües

-gua becomes -gües aigua-aigües (in order to preserve the [gua] sound

-tja becomes -tges platja-platges

-ça becomes -ces caça-caces