Help from native speaker, please

I would say incident, but I think either could be right.

Then again, I'm no expert on this kind of stuff.

‘Incident’ in that context I thought was a common noun as it’s a happening in the same way you could say there was a fair in town or a bus in town

An "incident" is an instance of something happening; an event or occurrence. So incident would be correct in your example.

"Incidence" is an amount or value - the occurrence, rate, or frequency of something e.g. there is an increased incidence of violent crime.

Hope that makes sense and is correct!

There was an incident in town and the police were called, is correct. 'Police' is a plural noun, incident is a singular noun.

Thank you. I'm helping a German student of English. She has written: There was an incident in town and the police was called.

I'll leave 'incident' but change 'was' to 'were'

Incident is a noun

Police is a collective noun; collective nouns can be treated as singular or plural. However, police is almost always treated as a plural noun.

Ps you may use incident as an adjective but it is not as often used as such.
In that sentence it is being used as a noun.

Agreed, There was an incident (single noun for an occurrence of some sort) in town and the police (plural) were called You would use the (single noun) incidence for the (non-specific) quantity or number of cases or occurrences of something, a bit like frequency .

The incidence of measles in the population has increased considerably lately.

In photography or graphics you could refer to - the light falling onto and illuminating an object as the incident light. It is the light as is comes from the light source. The light that bounces back from an object to the eye is the reflected light and defines the colour and how light or dark the object is. It has been influenced by the surface of the object.

Thank you all very much. The fog is clearing! It seems then that we don't come across 'incident' as an adjective very often.

Nowadays, most of us treat police as plural and would both say and write the police were called, but anyone my age was taught as already mentioned that it is a collective noun and that one should say and write "the police was called".

It is illogical, but I think we have to accept that "police" and some other collective nouns are being treated as plural these days, while others are not.

I myself would tend to say the police were called, but that a flock of sheep was in the road!

Like others who have commented, I would usually regard incident as a noun, and cannot think when I last used it as an adjective or incidence, which is always a noun, either for that matter.

It might be wise in any case to advise your student that active forms are generally easier to understand than passive forms. So it would be clearer to say ‘there was an incident in town and someone called the police’.