Usually words ending
in -a are feminine.
Exceptions are words
borrowed from Greek (-ema, -ama
and -oma, e.g. um telegrama, um
sistema) and professions
(um jornalista 
).
Words ending in -ção,
-são, -stão or -gião
are feminine (e.g. a televisão)
Words ending in -dade,
-gem, -ie or -tude
are feminine (uma mensagem, uma cidade)
Usually just add -s
when the words ends with a vowel unless the word ends
in -ão (os carros, as praias).
If the word ends in
consonant other than l or m,
add -es (o português, os
portugueses*).
If the word ends in s
and the stress is not on the last syllable, the
plural is the same as singular (o lápis, os lápis).
If the word ends in m,
the plural is formed with -ns (o
homem, os homens).
If the word ends in -ão,
the plural is usually formed with ending -ões
(a televisão, as televisões), in some cases -ães
(o cão, os cães) and sometimes -ãos
(o irmão, os irmãos)
If the word ends in -al,
-el, -ol or -il,
the plural is correspondingly formed with endings -ais,
-éis, -óis and -is.
If the -il ending is not stressed,
the plural is formed with ending -eis
instead of -is.
*The way the word is pronounced changes
when -es is added to make the plural, thus the
circumflex ("^") is missing in plural.
Under construction