You think English has weird rules for plurals, Try Finnish
General rule: Add "t" ( this works because so many Finnish words end a vowel)
Example:
Koira - dog
Koirat- dogs
However, if the last syllable of the Word has k,p,t,kk,pp, or tt, then the following rules apply
K is removed from the word
Sika -pig
Siat -pigs
P becomes V
Lupa- permission
Luvat - permissions
T becomes D
Äiti - mom
Äidit - moms
With the double consonants, you drop one of them
Takki - jacket
Takit -jackets
Noppa- dia
Nopat -dice
Kettu -fox
Ketut -foxes
Some words end with "nen", in this case, the "nen" is replaced with "set"
Nainen - woman
Naiset - women
With words ending with "i", its depends on whether is is a "new" or "old" word.
An example of a new word:
Paperi - paper
In which case you use the general rule and add t
Paperit -papers
With old words, the i is changed to e
Meri- sea
Meret-seas
Siipi -wing
Siivet - wings ( note the P to V rule)
Old words ending in "si", "si" becomes "de"
Vuosi - year
Vuodet - years
If the word ends in e, add another e
Perhe - family
Perheet- families (note on double vowels: it doesn't change the vowel sound, you just carry it a bit longer)
and these aren't totally inclusive of all the rules, for example
Mies - man
Miehet - men
Poika - boy
Pojat - boys
To add to the fun, the plural case is also applied to the adjective describing the noun.
New room - uusi huone
New rooms - uudet huoneet
And this doesn't touch on the other cases
For example
These boys will chase those boys - Nämä pojat jahtaavat noita poikia.
With both pojat and poikia being "boys" but one is the subject and the other the object.