Just a couple of sentences.

Tusen takk! What do you think, is it better to learn maskuline and feminine like one from the beginning or not? I mean, how unnatural is it to still use the feminine? Because in the book there is a clear distinction everywhere.

When you say masculine and feminine, are you refering to noun genders? If so, then yes, it's important to learn genders as early as you can. Knowing the differences of these genders totally rely on memorization. Don't let yourself be frightened of this, though. Once you start learning common word's gender and are able to use them in different context, it will become much easier.

Some words will allow both feminine and masculine definitive forms, and learning both of them is important since they are both very common to use. Even I sometimes switch between "ei flaske, en flaske" (a bottle), "ei uke, en uke" (a week), "ei klokke, en klokke" (a clock). They mean the exact same thing, although feminine forms usually sound more plain, and masculine forms usually sound more clinical (like some neutral text on an academic level).

But as mentioned, both versions are accepted, and both are very common. Learning both of them will aid you in recognizing and understanding things easier.

Thanks) I understand, that you have to learn genders, I've been learning German for a very long time) I've just heard somewhere, that people tend not to use feminine in Norwegian, if it's common gender. But I guess it's this difference between the common and academic speech you were referring to.