The police must fine you $100

[Winwin2011]

1. If you throw rubbish, you will be fined $100.
2. If the police saw you (throwing rubbish), they must fine you $100.

Are the above sentences natural?

Thanks.

[emsr2d2]

The second isn't very natural. Most cities don't have laws against people being being spotted doing something by the police. The first is definitely more natural, though in the UK, we refer to "dropping litter", not "throwing" it. The verb "to litter" also exists.

On a sign in a British park or street, you might see:

Fine for littering - £100.

[Tdol]

Also, the tenses don't match in the second- it would be OK with see/will fine or saw/would fine. I don't like must there- you could make a context where it would be possible, though.

[SoothingDave]

I don't think the police "must" do so. They wouldn't, for example, stop the pursuit of a bank robber to issue you a fine for littering.

[Tdol]

No, but you could create a context where it might work- the police haven't been enforcing the law, but they have received orders from above to do so....