Quick answer
Fortuito describes an unforeseen chance event; aleatório describes lack of a determined pattern or a random selection method.
A reliable test is to identify the job of the word before translating from English.
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What each word does
fortuito
fortuitous or unforeseen
Use fortuito for happening by chance or outside reasonable prediction and control.
aleatório
random
Use aleatório for selected or occurring without a determined pattern, often by a random procedure.
How to choose
- fortuito: Choose fortuito if the sentence points to happening by chance or outside reasonable prediction and control.
- aleatório: Choose aleatório if the sentence points to selected or occurring without a determined pattern, often by a random procedure.
Examples
O encontro fortuito mudou o rumo da pesquisa.
The chance encounter changed the course of the research.
A amostra foi escolhida de modo aleatório.
The sample was chosen randomly.
Common trap
A random sample is aleatória by design, not fortuita. A chance encounter is fortuito even without a statistical procedure.
Practice it in context
Pick one sentence from your real life and write it twice: once with fortuito, then once with aleatório. If both versions feel possible, name the meaning shift.
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