The quick answer is that you should always tell the reader where the information is from when paraphrasing what someone has written or said. However, there is no black and white answer that goes for every occasion and all subjects.
Common knowledge that won't be refuted usually doesn't need a reference. This can for example be dates for well-known events or other commonly known facts such as the fact that Swedish is the official language in Sweden. Facts you can expect that most people know without looking in an encyclopedia.
The fine line of what is common knowledge and what is specialist knowledge within different subject fields, and therefore requires a reference, is sometimes difficult to distinguish. If you are uncertain, it is a good idea to check with your teacher. However, the general idea is that all information that you have taken from another source requires a reference.
You should always have the reader as your frame of reference. It should be easy for the reader to understand what part of your text is connected to the different sources and what possibly is your comments or conclusions.
Strive to make the text as readable as possible and thus insert the reference smoothly. The reader should not have to stop reading to read out your reference.
If you have a longer paragraph that comes from the same source, the best practice is to insert the reference at the beginning of the paragraph as a narrative citation. It is often enough to include the reference once, but if the citation is long it could be a good idea to make the reader aware of the fact that it is still the same source that is being referenced. This is also a good way to, if needed, clarify that is not your own opinion that is presented. You do this by using reporting verbs together with the author. If the paraphrase continues into a new paragraph, reintroduce the whole citation.
There are a lot of different reporting verbs to choose from. Vary the verbs and choose according to what is fitting in the context.
