Need advice if the sentence is grammatically correct. Thank you, teachers

[Help Wanted]

Are the following sentences grammatically correct? Please advise, thank you, teachers

1. There will be no trees left if we cut down too many of them.
2. There will be less trees if we produce too much paper.
3.When a snake bit Lily, she was reading in the garden.
4. Jane was washing her car while the postman came to the gate.

PLUS:

5. We cannot swim (at/in/on ??? which one should I use? ) the beach if the beach is dirty.

1. ok grammatically, questionable content-wise ;)
2. FEWER trees
3. ok, but i would probably say "A snake bit Lily while she was reading in the garden."
4. ok
5. AT (you can't swin in sand or on sand)

[RonBee]

As AintFoolin said, the first one would make more sense if it was phrased thusly:

There would be no trees left if we cut them all down.