Study the information about prepositions on, in with other uses.

1. Remember these phrases:

on business
on a trip
on a tour
on a cruise
on holiday
I am going to Portugal on business.
On strike
On a diet
On fire
On the whole (=general)
On purpose
Joy has problems at work, but on the whole, she likes her job.
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But! We use by with: by mistake, by accident, by chance
2. Expressions with the preposition in:

In the sun, rain, shade, dark, bad weather
Write in ink, biro, pencil
In words, in figures, in block letters
Pay in cash
Be/fall in love
In someone’s opinion
In my opinion, the concert was wonderful.