In English you make most plurals by adding -s: one animal, two animals. Portuguese words that end in -l don't work that way. The -l drops out and -is takes its place, so animal becomes animais, papel becomes papéis and azul becomes azuis. This applies to nouns and adjectives alike, and you'll meet it from your very first lessons, with words like hotel, jornal and fácil.
English speakers tend to trip up in two ways. The first is simply adding -s by habit, which gives forms like animals or hotels that Portuguese doesn't have. The second is the small print. Sometimes an accent appears in the plural, sometimes it doesn't, and words in -il split into two groups. The good news is that it all comes down to where the stress falls.
The basic move is always the same: take off the -l and add -is. The vowel before it decides the exact shape:
-al becomes -ais: animal, animais.
-ul becomes -uis: azul, azuis.
-el becomes -éis when the stress is on that last syllable (papel, papéis). It becomes -eis, with no accent there, when the stress falls earlier (túnel, túneis).
-ol becomes -óis when stressed: sol, sóis.
-il becomes -is when stressed (funil, funis). It becomes -eis when unstressed (fácil, fáceis).
A handy rule of thumb: if the word already has a written accent earlier on, like fácil or túnel, the stress isn't on the ending. The plural then ends in plain -eis, and the original accent stays where it was.
The words
Words in -al are the most common group, and they're completely regular: animal, hospital, jornal, metal, sinal, canal, catedral, festival, material, capital and casal. Adjectives such as nacional and especial also follow it. Azul is the everyday example of -ul.
Há muitos animais perto dos canais.
(There are lots of animals near the canals.)
Comprei dois jornais no quiosque.
(I bought two newspapers at the kiosk.)
Stressed -el and -ol add an accent in the plural: papel, hotel, anel, cruel and fiel give -éis. Sol, lençol, farol, caracol, espanhol, girassol and cachecol give -óis.
Os hotéis têm lençóisazuis.
(The hotels have blue sheets.)
Preciso de mais papéis para a impressora.
(I need more sheets of paper for the printer.)
Os faróis do carro não funcionam.
(The car's headlights don't work.)
Unstressed -el gives plain -eis. This covers túnel, nível and the huge family of words in -vel: confortável, automóvel, telemóvel, móvel, possível, impossível, agradável, saudável and disponível.
Os exercícios são fáceis, mas os testes são difíceis.
(The exercises are easy, but the tests are hard.)
Os portáteis são muito úteis no trabalho.
(Laptops are very useful at work.)
Watch out
The most common slip is mixing up the two -il groups. Funil is stressed at the end, so its plural is funis. Fácil is stressed on the first syllable, so it becomes fáceis, not fácis. Say the word aloud: if your voice lands on the -il, you just need -is.
Don't add an accent where the singular had none on the ending. It's túneis and níveis, not túnéis. And keep the accent a word already has: possível becomes possíveis, and difícil becomes difíceis.
Remember that adjectives agree too, so a plural noun pulls the -is ending onto its adjective:
Os anéis são feitos de metais diferentes.
(The rings are made of different metals.)
A very small number of short words break the pattern. The best known is mal (evil), whose plural is males. You'll meet it rarely, so treat it as a one-off.
Summary
For words ending in -l, swap the -l for -is: animais, azuis. Stressed -el and -ol take an accent (papéis, sóis). Stressed -il becomes -is (funis). When the stress falls earlier in the word, the ending is plain -eis (túneis, confortáveis, difíceis). Never just add -s.
How words ending in '-l' form their plural