The semicolon is probably the most misused punctuation mark in English. What makes it even more difficult to use is that different languages use it in a different way. It is also used in computer programming.
Here is a site that explains it in a simple, graphic way. It calls the semicolon ‘the most feared punctuation on earth’ which is not a total exaggeration.
My tip would be – if you are not comfortable with semi-colons then avoid them. You can always rewrite a sentence so that you do not need to rely on punctuation to keep the syntax under control. The rules about main clauses and subordinate clauses (huvudsatser och bisatser) are more or less the same in Swedish as in English so shorter simpler sentences should not need a great deal of punctuation to make them understandable. There should usually not be more than one main clause in a sentence. It is a myth that simple sentences are ‘baby’ sentences. It is actually harder to write a simple sentence than a complicated one.
A good book is English for Journalists by Wynford Hicks. He explains punctuation very well and it is a good reference book if you are working with important lengthy texts.