What Is Their Main Difference?

The main difference between 'when' and 'once' is that 'once' expresses a more urgent situation than 'when.'

Parts of Speech

'When' and 'once' have different functions in the English language. We have summarized their parts of speech in the table below for better understanding:

If you pay attention to the table illustrated below, you can see that 'when' and 'once' can be used as conjunctions. Conjunctions are categorized into different groups. 'When' and 'once' are categorized as subordinating conjunctions.

Example

When I find her, I will let you know.

Once I find her, I will let you know.

Talking about Time

'When' and 'once' both express time. They are used to show that something happens as soon as something else occurs.

Example

I was sleeping when he called me.

I will come back once I have finished this book.

Are They Interchangeable?

As conjunctions 'when' and 'once' express the same meaning= 'as soon as,' and 'after.' Note that 'once' is more emphatic than 'when.'

Example

Once you sign the contract, there is no way out.

When you sign the contract, there is no way out.

Comments

(2)
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Metia Bethell
Feb 2024
They are not directly interchangeable, otherwise you could use the first example 'I was sleeping once he called me' which is clearly incorrect. 'When' here expresses something that is happening at the time, not afterwards. Furthermore, the next example 'I will call once I finished this book' is not grammatically correct as 'once' here (or even when, if used) should be followed by the present perfect 'have finished'. 'I will call once I have finished this book.'
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Langeek
Feb 2024
Thanks for the feedback! You are right, we will apply the necessary corrections.
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