why

1.adverbA1

Definition

used when you want to know the cause, purpose, or explanation of an action, event, or situation — asking what made something happen or what someone's intention was.

Example

Why did Eitan decide to move back to Japan after living in London for ten years?

direct question: why + did + subject + verb

Reema asked her brother why he had not called their mother on her birthday.

reported question: asked + why + subject + verb (normal order)

Common collocations

ask why
wonder why
know why
explain why
understand why
tell me why
why not

Synonyms

how come
informal equivalent used in casual conversation; does not require auxiliary inversion ('How come you left?')
for what purpose
more formal and specific about intention rather than general cause

Grammar Patterns

why + do/does/did + subject + verb?

why + be + subject?

reported: ask/wonder/know + why + subject + verb

Usage Note

In everyday spoken English, 'why' is very frequently used alone as a one-word question after a statement (e.g. 'I am not going.' 'Why?'). It also pairs with 'not' to make suggestions: 'Why not try the new café?'

Common Mistakes

Why you did that?

Why did you do that?

In direct questions, 'why' must be followed by an auxiliary verb (do/does/did/is/are) before the subject.

I do not know why is she upset.

I do not know why she is upset.

In indirect questions after words like 'know' or 'wonder', use normal subject-verb order, not question order.

Idioms

why ever

idiom

Used in questions to show that the speaker is surprised or upset.

Why ever did you paint the door bright pink?

Why ever would Maya leave her job so suddenly?

why not?

idiom

Used to make a suggestion or to agree with one.

Why not ask your teacher for help?

Diego suggested a hike, and I said, 'Why not?'

1.conjunctionA2

Definition

introduces a clause that gives the cause or purpose of an event, explaining what made it happen or what someone intended — used after verbs like 'know', 'explain', 'understand', and 'ask'.

Example

Christopher explained why he had missed the team meeting — his flight was delayed by six hours.

explain + why + clause (past perfect)

That is why Kenji always checks the oil level before a long drive.

that is why + clause (result structure)

Common collocations

know why
explain why
understand why
see why
figure out why
that is why
show why
ask why

Synonyms

for which reason
more formal; often used in academic or legal writing
the reason that
interchangeable in most contexts, though 'the reason that' is slightly more explicit

Grammar Patterns

know/understand/explain/see + why + clause

that is why + clause

the reason is why + clause

Usage Note

When 'why' introduces an embedded clause (after 'know', 'explain', 'wonder', etc.), the clause keeps normal subject-verb word order — do not use the inverted question structure. Compare: 'Why did she leave?' (direct question) vs. 'I know why she left.' (embedded clause).

Common Mistakes

I wonder why did he say that.

I wonder why he said that.

After 'why' in an indirect question, the subject comes before the verb, not after an auxiliary.

2.conjunctionB1

Definition

used after a noun such as 'reason' or 'cause' to link it to a clause that explains the content of that reason — for example, 'the reason why they left' means 'the reason for which they left'.

Example

The main reason why Élise moved to Berlin was the opportunity to work in film.

the reason why + clause

There are several reasons why the old bridge needs to be replaced before winter.

Common collocations

the reason why
reasons why
a good reason why
the cause why

Synonyms

for which
the formal equivalent; 'the reason for which they left' is more formal and less common in speech

Grammar Patterns

the reason why + clause

the reasons why + clause

(a/the) cause why + clause

Usage Note

Some traditional style guides discourage 'the reason why' as redundant ('reason' already contains the idea of 'why'), but this construction is extremely common in both spoken and written English at all levels, and most modern dictionaries accept it as standard. Removing 'why' ('the reason they left') produces a cleaner style that is also correct.

Common Mistakes

The reason why because he was tired.' (double cause marker).

The reason why he left was that he was tired.

Do not use 'because' together with 'the reason why' in the same clause.

Word family

wherefore
adverbC1

Synonym discussion

Sense: adverb/1

'Why' is the default, neutral word for asking about a reason or cause, used at all levels of formality. 'How come' is its informal counterpart: it does not need auxiliary inversion ('How come you left?' not 'How come did you leave?') and is common in casual American and British speech but avoided in writing. 'For what purpose' is more formal and specific — it asks about intention rather than general cause, so it fits contexts like legal questioning or academic debate ('For what purpose was the data collected?'). 'Wherefore' is archaic; modern readers mostly know it from Shakespeare's 'Wherefore art thou Romeo?' (which actually means 'why,' not 'where'). Learners at A2–B1 should master 'why' and recognise 'how come'; 'for what purpose' and 'wherefore' are passive vocabulary for reading.

why ↗how come ↗for what purposewherefore

Common collocations

Nouns

reason why
cause why
whys and wherefores

Phrases

the reason why
the whys and wherefores of
why not
that is why

Etymology

Old EnglishGermanicEnglish

'Why' comes from Old English 'hwȳ', the instrumental case of 'hwæt' (what), meaning 'by what cause or means.' It is related to Old Saxon 'hwī' and Old High German 'hwiu'. The interrogative and relative uses developed very early in Old English. The exclamation use ('Why, I never!') emerged in the 16th century as an interjection expressing surprise or indignation, probably a natural extension of the questioning form used rhetorically.

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why

why — adverb

1. used when you want to know the cause, purpose, or explanation of an action, even

Grammar Patterns

Usage Note

Common Mistakes

why ever

why not?

why — conjunction

1. introduces a clause that gives the cause or purpose of an event, explaining what

Grammar Patterns

Usage Note

Common Mistakes

2. used after a noun such as 'reason' or 'cause' to link it to a clause that explai

Grammar Patterns

Usage Note

Common Mistakes

why — exclamation

1. said at the beginning of a sentence to show that you are surprised, mildly annoy

Grammar Patterns

Usage Note

Common Mistakes

why — noun

1. the causes, explanations, or reasoning that lie behind an action, decision, or s

Grammar Patterns

Usage Note

Common Mistakes

Word family

Synonym discussion

Common collocations

Nouns

Phrases

Etymology

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