Hi my friend, the first sentence you said doesn't exist "quand il plent", I think you meant "quand il pleut" which is "when it rains".
For the two other sentences, it's true that the N is not pronounced because AN or EN are a special sound ! You already know that OU for example is different from O or U, and you could say that the U of OU is not pronounced : it's the same here with the N.
You will have to learn 3 new sounds with the N : ON, AN and UN. They are very special and you will find them only in french.