My Grammar and I (Or Should That Be 'Me'?): Old-School Ways to Sharpen Your English

One sheep, two sheep…
The following nouns take the same form whether they are singular or plural. So
what would Sherlock Holmes have to say about that?
aircraft kudos
series
cannon means
shambles
deer
offspring
sheep
haddockpremises
*20
species
†21
trout
Take your pick…
All self-respecting pedants know about Latin plurals ; a smattering (and rapidly
diminishing  number)  of  older  ones  know  about  Greek,  too.  But  here  a  little
learning  can  be  a  dangerous  thing,  because  it  is  easy  to  assume  that  a  word
ending in -us is Latin second declension, and therefore has a plural ending in -i :
abaci  ,  cacti  ,  incubi  ,  succubi  .  Then  along  comes  a  word  such  as  platypus  ,
whose  origins  are  Greek  and  whose  plural  is  strictly  speaking  platypuses  ,  to
catch you out just when you thought you were being clever .
A number of words ending in -on are derived from Greek neuter nouns and have
a plural form ending in -a : criterion/criteria , phenomenon/phenomena etc.
Some words ending in -is are Latin third declension in origin and have a plural
form -es : crisis/crises , thesis/theses (pronounced -eeze as in cheese or sneeze ).

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And then there are those that have decided to ignore their classical background
altogether  and  allow  us  to  choose  between  two  (or  even  more)  plural  forms,
some of them rather suspect: