Meaning of when in English

He placed a bet on his team when he knows they're likely to lose.
They claimed to know each other when in fact they've never met before.
It was good of you to offer me a lift when I know you don't like driving at night.
It was thoughtless of you to cook steak when you know I'm a vegetarian.
She says she enjoys watching horror films when in fact she gets really scared by them.

Thesaurus: synonyms and related words

Linguistics: connecting words which express a contrast

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Grammar

If or when?

We use if to introduce a possible or unreal situation or condition. We use when to refer to the time of a future situation or condition that we are certain of: …

When

When is a wh-word. We use when to ask questions, as a conjunction and to introduce relative clauses. …

When as a question word

We can use when to ask for information about what time something happens: …

When as a conjunction

When as a relative pronoun

We can use when as a relative pronoun in relative clauses: …

Since when?

We can use since when to ask at what time something began. We often use it as a response when we are surprised that something has begun: …

When or if?

We use when to refer to a future situation or condition that we are certain of, whereas we use if to introduce a possible or unreal situation. …

When or since?

We use when to mean ‘(at) the time that’. We use since to refer to a particular time in the past until another time or until now: …

When: typical errors

…

(Definition of when from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

when | American Dictionary

when

conjunction, adverb [ not gradable ]

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/wən, hwən/

when conjunction, adverb [not gradable] (AT WHAT TIME)

at what time; at the time at which:

When is supper going to be ready?

When did the American Civil War begin?

I was just getting into the shower when the phone rang.

when

conjunction

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/wen, hwən/

when conjunction (CONSIDERING THAT)

considering the fact that:

I can’t really call myself a vegetarian when I eat fish.

when conjunction (ALTHOUGH)

despite the fact that:

He says he hasn’t got any money when the truth is he’s got plenty.

(Definition of when from the Cambridge Academic Content Dictionary © Cambridge University Press)

Examples of when

when

Selective participation could be an important source of bias when investigating seroprevalence of vaccinepreventable diseases given the low participation rate in some age classes.

From the Cambridge English Corpus

More than one membrane was used for each sample when needed.

From the Cambridge English Corpus

These examples are from the Cambridge English Corpus and from sources on the web. Any opinions in the examples do not represent the opinion of the Cambridge Dictionary editors or of Cambridge University Press or its licensors.

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When venturing beyond empirical data, scientists often bring in unnecessary and unnoticed conceptual baggage which can obstruct constructive consensus.

From the Cambridge English Corpus

Needed to make a high vowel non-back (front) when an onsetless vowel is word-initial.

From the Cambridge English Corpus

Required to yield denasalization when an onsetless vowel is word-initial.

From the Cambridge English Corpus

The other is that such a system generalizes locally when there are many similar elements that share a property.

From the Cambridge English Corpus

When the subject follows the verb, there is no number agreement.

From the Cambridge English Corpus

When the subject is in front of the verb, full agreement appears, see (6).

From the Cambridge English Corpus

Note, though, that these verbs can take the nasal prefix when the applicative/causative suffix -ke is present.