Linking two nouns
こんにちは、皆さん。私の問だいはこれです:
Everytime I link two nouns do I have to put の particle as for example between れんしゅう and 時間
E.g.: 今れんしゅうの時間です。じゃあ、手紙を書いてください。
It's practice time. So, write a letter please.
This is just an example, because sometimes I find difficult to link two nouns. Is there any rule or something like that, that makes easier to know if I have to use a particle or not?
ありがとうございます!