How do you say it is not ok to do something in a National Park in Japanese? e.g. It is not ok to litter in a national park.For example, "It is not...

The answer from dymatsuoka is a perfectly good one, but I would like to offer another way to translate/express this sentiment.

Depending on the situation and the person being addressed, Japanese people often like to mould the way they say things to make it as nice and unassertive as possible. As a result, we get phrases like the one above.

If you click on the first link in the references section, you will see a sign which reads:

「ゴミは各自でお持ち帰りください」

or in roman letters (using wa for the ha particle)

"Gomi wa kakuji de o mochi kaeri kudasai"

This translates to:

"Please take your litter (home) with you"

To make it even more friendly and polite (and a bit shorter), you can change the above sentence to,

「ゴミは持ち帰りましょう」

"Gomi wa mochi kaeri mashou"

Which gives it a feeling of,

"Let's all be sure to take our litter (home) with us"

I see this phrase used most often in public parks so I recommend it!

Thanks for the question!