There are words, like potato where the plural form ends with es (potatoes). In which cases does a word have a plural ending with es, but the singular doesn't end with e?
Is there a rule for that, or do words like potato have an irregular plural?
Wikipedia gives a pretty good overview of the rules. Based on the information there (don't cite it in a paper, but it's good enough for our purposes), nouns ending in -o are accompanied by nouns ending in a sibilant sound, which is far beyond the scope of this answer. But at any rate, here's the relevant sections from the Wilipedia article on English plurals:
Where a singular noun ends in a sibilant sound —/s/, /z/, /ʃ/, /ʒ/, /tʃ/ or /dʒ/— the plural is formed by adding /ɨz/. The spelling adds -es, or -s if the singular already ends in -e:
kiss kisses /ˈkɪsɨz/
phase phases /ˈfeɪzɨz/
dish dishes /ˈdɪʃɨz/
With nouns ending in o preceded by a consonant, the plural in many cases is spelled by adding -es (pronounced /z/):
volcano volcanoes or volcanos
However many nouns of foreign origin, including almost all Italian loanwords, add only -s:
So in summary, add -es if: