1,Here there are many interesting places to visit
2 the river , which goes through the forest ,is 5000 km long
3 The students present all attended yesterday's meeting
4 it's very clear that this film is not suitable for teenagers
5 whether the art exhibition can remain open to the public on time is a problem (whether 引导主语从句（在句首）)
或 it's a problem whether the art exhibition can remain open to the public on time or not
6 we found that it's necessary to go over what we learned before the exam