Since I believe every student of Portuguese as a second language has called me Marcelinho at least once, I thought this might be a useful article to write about – when to form diminutives using -inho and when to do it using -zinho. These are general general guidelines, though, so no blaming me for exceptions found along the way.
1. One-syllable words and words stressed on the last syllable add -zinho:
o pé > o pezinho
a mão > a mãozinha
o pão > o pãozinho
o chá > o chazinho
o café > o cafezinho
o avô > o avozinho
a avó > a avozinha
o casal > o casalzinho
o barril > o barrilzinho
Marcel > Marcelzinho
2. Words stressed on the penultimate or antipenultimate syllable that end in -ã, -ão, in a diphthong or a hiatus also add -zinho:
o órfão > o orfãozinho
a órfã > a orfãzinha
o índio > o índiozinho
a lua > a luazinha
a sereia > a sereiazinha
2a. Possible exceptions/alternative forms made common through usage and that work as the words in case 3 (below) exist, though:
a praia > a prainha
o rádio > o radinho
a rua > a ruinha
Emília > Emilinha
3. Words stressed on the penultimate syllable that don’t fall in case 2 drop any possible final vowel they have and add -inho:
o livro > o livrinho
a moça > a mocinha
a parede > a paredinha
o filho > o filhinho
Marcelo > Marcelinho
3a. Exceptions/alternative forms exist, though, and some such words add -zinho when the diminutive form wouldn’t be euphonic or in the literary language:
a ave > a avezinha
a ilha > a ilhazinha
o gole > o golezinho
4. Words ending in -z, -s, or -s + vowel add -inho:
o nariz > o narizinho
a rosa > a rosinha
o português > o portuguesinho
o rapaz > o rapazinho
Luís > Luisinho
5. The plural of words (other than those of cases 3 and 4) above drop the final -s and then add -zinho:
os pés > os pezinhos
as mãos > as mãozinhas
os pães > os pãezinhos
os avós > os avozinhos
os casais > os casaizinhos
os barris > os barrizinhos
as luas > as luazinhas
as aves > as avezinhas
os órfãos > os órfãozinhos
5a. The plural of words of cases 3 and 4 above just add a final -s to the singular diminutive:
os livros > os livrinhos
as paredes > as paredinhas
os narizes > os narizinhos
as rosas > as rosinhas
5b. Some words have popular plural forms that are either colloquial or made valid through usage:
as mulheres > as mulherzinhas
as flores > as florzinhas
as pastoras > as pastorinhas
6. Many words have irregular/erudite diminutives, often stemming from Latin, and sometimes they’re synonyms of the regular ones, add specific different nuances, or form different words altogether (that in turn may have their own diminutives as well):
o rio > o riacho
o fio > o fiapo
a casa > o casebre (gender change)
a mosca > o mosquito (gender change)
o mosquito > o mosquitinho
o jornal > o jornaleco
o lugar > o lugarejo
a chuva > o chuvisco (gender change)
a maçã > a maçaneta
a caixa > o caixote (gender change)
o caixote > o caixotinho
And it’s probably never enough to remember that diminutives no only give the idea of a small/little version of the original word, but may also be used affectionately, derogatorily, to express slow or fast actions (as when used with adverbs), or small quantities, etc.