Short answers, pulled from the story.
The first Swedish translation was made by Åke Ohlmarks between 1959 and 1961 under the title Sagan om Ringen. It remained the only Swedish version for forty years until Erik Andersson's prose translation and Lotta Olsson's rendering of the poetry were published in 2005.
Tolkien called Ohlmarks "a conceited person" in a 1957 letter to his publisher Rayner Unwin and judged his translation worse than Max Schuchart's Dutch version. Tolkien identified numerous errors, including Rivendell rendered as Vattnadal (Water-dale) and the Ent Quickbeam mistakenly rendered as Snabba solstrålen (Swift Sunbeam), ignoring the convention that all Ents have tree-related names.
Tolkien wrote the Guide to the Names in The Lord of the Rings in 1967 as a framework for translating personal names and place names. It was produced partly in response to Åke Ohlmarks's Swedish translation and Max Schuchart's Dutch translation, giving multiple examples of errors from Ohlmarks's text.
Ohlmarks rejected all criticism, stating in his 1978 book Tolkiens arv that his intention had been to create an interpretation of Tolkien, not a straight translation. He did not revise the work in response to reader complaints.
Erik Andersson's 2005 translation follows Tolkien's 1967 naming guide closely and keeps much closer to the length and style of the original prose. Where Ohlmarks expanded a twenty-word Tolkien sentence to forty-two words, Andersson's equivalent runs to twenty-four. Andersson retained a few of Ohlmarks's well-established name choices, such as Vidstige for Strider and Fylke for the Shire, but replaced mistranslations like Vattnadal (Water-dale for Rivendell) with Riftedal.
When The Silmarillion was published in 1977, Christopher Tolkien, Tolkien's son and literary executor, consented to a Swedish translation only on the condition that Ohlmarks have nothing to do with it. The translation was carried out by Roland Adlerberth.