be used to meabn exist/happen/take place

[Saneta]

When be is used to mean exist/happen/take place the there construction is necessary:
There is a mistake/ There are mistakes in this translation.
These sentences couldn't be rewritten:
A mistake is/Mistakes are...etc.

So , why we can say:
A policeman is at the door?= because is=exists??

Many thanks

[Dozy]

This appears to be an exception to that supposed rule. Similarly,

"A cow is in the middle of the road."
"Angy crowds are in the town square."
"Mistakes are everywhere."
etc.

Explaining exactly why some sentences like this work and some don't seems quite tricky.

[Saneta]

Hi, Dozy, thanks for your opinion.
what do 'angy crowds' mean? angy?

thanks

[Dozy]

Oops, that was a typo for "angry".