Example: I go to a bookshop. I = watashi wa go to = iku a bookshope = hon-ya Connecting them and add, Watashi wa hon-ya ni iku. (It's correct!) Adding polite expression "desu", Watashi wa hon-ya ni iku-desu. (It's false !) Because, verb and "desu" should not be connected directly. For connecting them, "no" is needed, Watashi wa hon-ya ni iku-no-desu. (It's correct!) And to say it casually, Watashi wa hon-ya ni ikun-n-desu (ikun-desu). The rule above is not perfect law, but it applues in many cases.

Manu, I'll try to explain it. If you still don't get it, please ask me further. のです nodesu is a particle of the end of sentence. There are other several styles of the same function, such as だda, ですdesu, ますmass　etc. Sometimes they work in the same way, but also have individual tones and nuances. e.g. わたしのなまえはマヌです。watashi no namae wa manu desu. My name is Manu. わたしのなまえはマヌなのです。 manu nanodesu. The frist sentence is very common and it just explains your name. The second sounds something particular, which you need more information about the reason why you say like this. But still it is correct. わたしはみた。watashi wa mita. I saw (it). わたしはみたのです。 You're trying to say it in a highlighted tone because of some reason. You might explain it, because, e.g., you are a witness, or you try to confess.