In my experience there are several issues that causes problems for someone who are learning Norwegian with English as their primarily language, or someone who uses English as reference because their native language is too different.
Firstly, whenever you start to learn Norwegian you will have to learn the basic elements. Some prefer to learn Norwegian through a lot of not basic texts. This can seem great in the beginning. Instead of learning through the lame dialog found in basic book, you will learn through more interesting texts. In the beginning you might not care that you don't understand completely everything. Problem is that you will probably develop bad habits that can be difficult to shake later. Regular Norwegian classes follows the layout of the Norwegian Course outlined. Often I see these errors among those who have not learned Norwegian step by step:
1) Remember that Norwegian do NOT have -ing form in verbs. The present is, for regular verbs, simply made by adding -r. This is not among the more common errors, but it is a very serious mistake. Norwegian does have -ing words but they are describing the act of and are not verbs.
2) More common errors, but not as serious as the one mentioned in 1), are pronunciation errors. There are many special rules and sounds in Norwegian. E.g. "gj" usually turns into "j" when spoken. Commonly some letters are also silent in the end of words. Native Norwegians will generally be able to understand you if this is the only kind of mistake you are making. The problem, however, is that it will be very difficult to understand when Norwegians speak when the words don't sound exactly the way they look.
Bryggen in Bergen, Norway
3) The Third problem that is very common is the word order. It is not too difficult to learn the position of the verbs in Norwegian. That being said, it often become more difficult if you have developed a bad habit. In addition, this kind of error will make your Norwegian sound really amateurish.
I will hopefully include some further instruction about these elements later.