failvsfastWhat's the difference?

Spelling & Dictionary Insight

fail and fast form a confusable pair in the Spanish index, two distinct headwords that writers substitute for each other because they look alike, sound alike, or both. The pair differs by a single letter swap, which is exactly the edit distance at which substitution errors are most common: close enough that the eye skips over the difference, far enough that meaning fully diverges. Our composite confusion score for this pair is 47059, derived from the frequency rank of both members and their visual similarity.

Side-by-side the two words carry different dictionary signatures. fail is recorded at frequency rank #30,476, classified as averb, pronounced /feɪl/. fast is at rank #16,583, tagged as anadj, pronounced /fɑːst/. When the two words belong to different parts of speech, sentence grammar alone usually resolves the confusion; when they share a part of speech, only semantic context separates them, which is why the pair earns a dedicated lookup page.

Glosses for this pair are partially populated in our dataset, but the full side-by-side definitions above should still guide you to the right choice. Automated spell-checkers cannot flag confusable substitution because every member of the pair is a valid dictionary word, only the writer, or a grammar/context tool, can confirm that the chosen spelling matches the intended meaning. PlainSpell's confusable index exists precisely to make that contextual choice explicit.