Most plurals are formed simply by adding "s" to the root word
(bat/bats). But there are innumerable alternatives and variations.
Certain word endings such as "ch," "sh," "s," "z" and "x" require
an "e" before the final "s" to facilitate pronunciation. If you
hear this sound (as in lunches, bushes and boxes) use the "-es" ending.
Words ending in "y" form their plurals with -ies if the letter
before the "y" is a consonant (party/parties); if the penultimate letter is a vowel simply add an "s" (key/keys).
An "f" usually becomes a "v" in the plural (calf/calves). However,certain variant forms change neither spelling nor sound
(graph/graphs, giraffe/giraffes roof/roofs).
Some words ending in "o" (tomato, potato) add an "e" in the plural (tomatoes), but this pattern is by no means consistent, especially with
words, often in the context of music, that are derived from Italian
(piano /pianos, trio/trios).
Some Latinate words such as alumnus use Latin endings to form plurals (alumni) Note that media and phenomena are already plural; use the singular when appropriate:
One phenomenon of the last half of the twentieth century was the
dominance of television as a medium.
Solid compound words add plural endings to the very end of the
word (spoonfuls) while hyphenated compounds add them to the noun (mothers-in-law.)
Many plurals are utterly unpredictable (child/children) and simply have to be memorized.