The main intuition: whenever, or each time
In a repeated situation, when often means “each time”. The speaker is not guessing about one future event, but describing a usual or predictable relationship: event A occurs, and result B follows.
These sentences describe patterns, rules, how a device works and personal habits. The event may repeat often or only in particular circumstances; what matters is that the relationship is presented as typical.
When + Present Simple, Present Simple
When the sun goes down, it gets dark.
When the sun goes down, it gets dark.
This is a predictable natural relationship; when means “each time this happens”.
When I work late, I take the bus home.
When I work late, I take the bus home.
The speaker describes a repeated personal habit, rather than one particular evening.
The form of both clauses
Within this rule, both clauses use Present Simple. The when clause names the repeated event, and the main clause gives the typical result.
Remember the -s ending with he, she, it and singular subjects: When the sensor detects smoke, the alarm sounds. With I, you, we and they, use the base form.
When + subject + V1/V-s, subject + V1/V-s
- subject / S —
- The grammatical subject: who or what the clause is about; the verb agrees with it. In The book was written, the book is the subject, although someone else does the action.
- verb / V —
- A word for an action or state, such as work or be. The pattern determines its form.
- V1 / base form —
- The base verb without to: work, go, be.
When the battery gets low, the light turns red.
When the battery gets low, the light turns red.
Battery and light are singular, so use gets and turns.
The screen locks when you press this button.
The screen locks when you press this button.
The clause order has changed, but both verbs remain in Present Simple.
Clause order and the comma
The when clause can come first or second. If it comes first, put a comma after it. If the main clause comes first, a comma is usually unnecessary.
Changing the order does not change the basic relationship, but can change what the speaker puts in focus.
When A, B. / B when A.
When the lesson ends, we close our books.
When the lesson ends, we close our books.
The when clause comes first, so there is a comma before the main clause.
We close our books when the lesson ends.
We close our books when the lesson ends.
The when clause follows the main clause; no comma is needed.
When and if: similar, with different focus
For repeated or predictable situations, Cambridge allows both if and when with a present-tense form. When focuses on the timing of a regular or expected event; if focuses on a condition that may or may not be met.
Replacing one with the other therefore often keeps the main fact, but may slightly change the speaker’s viewpoint. Do not learn a mechanical rule that if always equals when.
When I feel tired, I go for a short walk.
When I feel tired, I go for a short walk.
When presents this as a familiar repeated situation.
If I feel tired, I go for a short walk.
If I feel tired, I go for a short walk.
If highlights the condition more strongly: the walk depends on whether I feel tired.
Where replacing one with the other is natural
The most natural contexts are general facts, predictable processes, instructions about a typical result and repeated habits. The condition should provide enough context for the reader to recognise a rule or repetition, rather than one chance event.
Words such as always, usually and every time, or a description of a device’s function, can help but are not compulsory. The sentence’s meaning itself should show regularity.
When you add hot water, the powder dissolves.
When you add hot water, the powder dissolves.
A predictable process that can be repeated is described.
If the printer has no paper, it stops.
If the printer has no paper, it stops.
This is a general rule, but if appropriately highlights the specific operating condition.
The boundary: a single future event
When can introduce a future time clause: When I finish work, I’ll call you. Here the speaker expects work to finish, and the main clause gives a future result. This is grammatical, but is not a zero conditional with two Present Simple forms.
If I finish early, I’ll call you means finishing early is possible but uncertain. In a single future situation, when and if therefore change the degree of expectation; do not replace them mechanically.
When the meeting ends this afternoon, I’ll call you.
When the meeting ends this afternoon, I’ll call you.
This is a correct future time clause: the meeting’s end is expected. It shows the boundary of our zero conditional.
If the package arrives today, I’ll text you.
If the package arrives today, I’ll text you.
If shows an uncertain future condition: the package might not arrive today.
Using the rule in your own speech
First identify whether you are describing a habitual repetition, a predictable process, a rule’s condition or a single future event. Then choose the focus: when for the regular occasion, if for the condition.
Check three things: is repetition or predictability clear, is Present Simple correctly formed in both clauses of the zero conditional, and does when avoid adding unwanted certainty about a single event?
When customers pay by card, the receipt prints automatically.
When customers pay by card, the receipt prints automatically.
The sentence naturally explains the system’s repeated operation.
If a customer needs help, I call my manager.
If a customer needs help, I call my manager.
If highlights the condition under which the employee performs the action.
Common mistakes
Incorrect:When water reach 100°C, it boils.
Correct:When water reaches 100°C, it boils.
The subject water is singular, so Present Simple needs reaches.
Incorrect:When you will press this button, the screen locks.
Correct:When you press this button, the screen locks.
In this repeated instruction, use Present Simple after when, rather than will.
Incorrect:When my sister gets home, she make tea.
Correct:When my sister gets home, she makes tea.
In the main clause, the subject she needs makes.
Incorrect:When people doesn't sleep enough, they feel tired.
Correct:When people don't sleep enough, they feel tired.
People is plural, so form the negative with don’t.