однажды vs когда-то
odnazhdy vs kogda-to — once/one day vs sometime
Register: neutral
What it really means
Однажды means once, one day, or on one particular occasion, often in storytelling. Когда-то means sometime in the past or future; context decides, and it can also mean “once upon a time.”
Examples in natural speech
Однажды я встретил актёра.
Once I met an actor.
Однажды всё изменилось.
One day everything changed.
Я когда-то жил здесь.
I once lived here.
Когда-нибудь мы встретимся.
We will meet someday.
Когда-то здесь был лес.
There used to be a forest here.
Common mistakes
Do not confuse когда-то with когда-нибудь: the latter is future/open possibility.
Memory tip
Однажды is one narrated occasion; когда-то is an indefinite time.
Frequently asked questions
How do I say once upon a time?
Когда-то or однажды.
Which is one event?
Однажды.
What is future someday?
Когда-нибудь.
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