Structure: ‘Time clauses’

What is a time clause?

A time clause is a dependent clause that tells us when something happened. It is introduced by a time conjunction such as when, before, after, while, or as soon as. On its own a time clause is incomplete — it always attaches to a main clause to show how two events are related in time.

The Denver runway security incident makes time clauses easy to see: a man climbed a fence, a sensor triggered, a plane began its takeoff roll — all within two minutes. Each event followed, or overlapped, another, and the conjunctions below are exactly what you need to describe that kind of sequence accurately.

The time conjunctions to know

• when — at the moment that: When the sensor triggered, cameras showed deer in the area.
• while — during the same period: Smoke filled the cabin while the crew brought the aircraft to a stop.
• before — earlier than: Before security could reach the fence, the man had crossed the runway.
• after — later than: After the aircraft stopped, all passengers were evacuated onto the taxiway.
• as soon as — immediately when: As soon as the engine fire started, the crew aborted the takeoff.
• until — up to the moment that: The runway was closed until investigators had finished their work.

Word order

The time clause can come first or second — both positions are correct:

Before security could respond, the man had crossed the runway.
The man had crossed the runway before security could respond.

When the time clause comes first, use a comma to separate it from the main clause. When it comes second, no comma is needed.

Tense after time conjunctions

After time conjunctions, we do not use a future tense — even when we are talking about the future. This is one of the most common learner errors at this level:

✗ The airport will reopen when the investigation will finish.
✓ The airport will reopen when the investigation finishes.

Use the present simple (or present perfect for completed actions) inside the time clause, never the future form.

Try these

Use the conjunction in brackets to join the two sentences into one. You may put the time clause first or second — both are correct. Remember: no future tense in the time clause.

1. A ground sensor triggered an alarm. The man approached the perimeter fence. (as soon as)
2. The aircraft struck the intruder. An engine fire started immediately. (when)
3. The cabin crew stopped the aircraft. They evacuated all 231 passengers and crew. (after)
4. The investigation will be completed. The airport will announce new security measures. (before)
5. The aircraft stayed on the taxiway. All passengers were evacuated. (until)
6. The runway will be confirmed clear. Flights will resume. (as soon as)

CEFR Level B1 / ICAO Level 4

Time clauses are one of several ways to build complex sentences in English. Once you are comfortable with them, Structure: ‘Relative clauses 2’ shows how to attach a different kind of dependent clause — one that describes a noun rather than placing it in time.