1) It's not easy to identify the right word to use: "a", "the", or nothing at all. 2) It's difficult to play the guitar while singing a song. (You need "while" here.) 3-a) It's impossible for me to ride a wave on a surfboard. (I think in American English / surf boarder talk, "ride a wave" sounds more natural) 3-b) It's impossible for me to be a surfer. (correct) 4) It's dangerous to walk alone in this area at midnight. (correct) 5) It's safe to fasten your seat belt when he is driving. (correct) 6) It's not expensive to go to that zoo. or Admission to that zoo is not expensive. 7) It's interesting to climb a tree in the woods. (this is grammatically correct but do you mean "It's fun to climb a tree in the woods."?) 8) It was nice to watch the sunset from your house. (I think we would actually say "from" instead of "at" here in American English) 9) It will be wonderful to go on a trip with you. (We say "go on a trip" in American English) 10) It's terrible to fall into the pond on such a cold night. (We would say "on such a cold night" in American English. It may be fine to say "at such a cold night" in British English. I don't know. Sorry.)