I guess that for me it either looks correct or it doesn’t.

Here are some observations gathered from the Web to explain this chaos. I’ll cover the consistent ones in this post and the next. Then we’ll look at the inconsistent ones.

For most words, add s.
   elephant becomes elephants
   stereo becomes stereos

For words ending with a hissing sound, add es.
   box becomes boxes
   church becomes churches

For words ending in a vowel plus y, add s.
   tray becomes trays
   keys becomes keys

For words ending in a consonant plus y, change y to ie and add s.
   enemy becomes enemies
   baby becomes babies

For words ending in is, change the is to es and add s.
   synopsis becomes synopses
   thesis becomes theses

For words ending in us, change the us to i.
   cactus becomes cacti
   fungus becomes fungi