Formerly vs Once

When to use each in English, with meaning, register, and examples.

Formerly

Formal Beyond 10,000 (less common) B2 adverb

Once

Top 1,000 (very common) A1 adverb

Most formal: Formerly Most common: Once

	Formerly	Once
Pronunciation	🇬🇧 /["/ˈfɔːməli/"]/ 🇺🇸 /["/ˈfɔːrmərli/"]/	🇬🇧 /["/wʌns/"]/ 🇺🇸 /["/wʌns/"]/
Meaning	in the past; before now	one time
Example	Namibia, formerly known as South West Africa	Once I finished my homework, I went out to play.
Register	Formal	Neutral
How common	Beyond 10,000 (less common)	Top 1,000 (very common)
CEFR level	B2	A1
Part of speech	adverb	adverb
Collocations	formerly a leader, formerly known, formerly used	once a week, once in a lifetime, once again, once more
Antonyms	currently, now	never
Common mistakes	Confused with 'formally' — 'formerly' relates to the past, while 'formally' relates to official contexts., Omitting it when describing past roles or states, which can lead to ambiguity.	Confused with 'once' for 'twice' or other frequencies., Using 'once' with present tense when referring to past events.
Usage notes	Use 'formerly' to describe something that was true in the past but is no longer the case. It is more formal than saying 'used to'. Avoid in casual conversations.	Use 'once' to refer to something happening a single time in the past. It's neutral and often used in storytelling. It wouldn't be appropriate in highly formal writing.

See it in real clips

Agatha Christies Poirot › S1 E1

Formerly in the employ of Mrs. Ernestine Todd

Formerly

Frequently asked questions: Formerly vs Once

What's the difference between Formerly and Once?

Formerly: in the past; before now Once: one time

Which is more formal: Formerly and Once?

Formerly is the most formal of these.

Which is more common: Formerly and Once?

Once is the most common in everyday English.

Which is more advanced: Formerly and Once?

Formerly is the highest level, at B2, on the CEFR scale.

Are Formerly and Once the same CEFR level?

Formerly: B2, Once: A1 on the CEFR scale.

What part of speech are Formerly and Once?

Formerly: adverb, Once: adverb.

Can you show an example of each?

Formerly: Namibia, formerly known as South West Africa Once: Once I finished my homework, I went out to play.

Can I use Formerly and Once interchangeably?

Not always. Formerly and Once are related and overlap in some contexts, but they differ in register, how common they are, and usage, so swapping one for another can change the meaning or tone. Check the differences above before substituting.