On-Phrases
Fixed expressions with 'on': on time, on purpose, on fire, on sale, on the phone, on business. These phrases typically describe a current state or ongoing activity.
On-Phrases in English
'On' in fixed phrases frequently describes a current state, an ongoing activity, or a scheduled condition. Learner corpus analysis identifies 'on purpose', 'on time', and 'on business' as among the most error-prone on-phrases at B1–B2 level, partly because related concepts use different prepositions ('by accident' vs 'on purpose') and partly because cognate languages use different prepositions for the same concepts.
Core On-Phrases
on purpose — He didn't break it on purpose — it was an accident.
on fire — Call the fire brigade! The building is on fire!
on sale — Everything in this shop is on sale — 30% off.
on the phone — She's on the phone right now — can she call you back?
on business — He's travelling on business this week.
On Time vs In Time
'On time' = punctual, at the exact scheduled moment. 'In time' = not too late, with enough time to spare. The distinction is covered in detail in the Confusable Pairs subtopic.
Common Mistakes
✗ The house is in fire! → ✓ The house is on fire!
✗ She's travelling in business. → ✓ She's travelling on business.