When vs. Once

'When' and 'once' are used to express time but what is their difference and when to use each of them? In this lesson, we will learn more about them.

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:

	When	Once
Interrogative adverb	✓	✗
Relative adverb	✓	✗
Subordinating conjunction	✓	✓
Adverb	✗	✓

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.

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.

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.'

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

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

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