Nielsen was the son of master carpenter Niels Nielsen (1883-1962) and his wife Ella Kirstine Rasmussen (maiden name) (1892-1979).[3]
He took his high school diploma at
Marselisborg Gymnasium in 1933, and in 1942 he obtained a master's degree in arts from
Aarhus University. Later, he held various teaching positions at the same university, until he became a professor of Nordic languages at
Odense University in 1966. However, he returned to his former university in 1972.[4]
He mostly devoted his attention to studying Danish dialects, especially the different dialects in
Jutland and the history of the Danish language.[5]
Sprogets opståen og udvikling (The Origin and Development of the Language).(Søndagsuniversitetet 88). Copenhagen: Munksgaard 1968.
Runerne på Rökstenen (The runic Letters on Rökstenen). Odense: Odense Universitetsforlag 1969.
Dansk dialektantologi (Danish Dialect Anthology). Volume 1: Østdansk og ømål. Volume 2: Jysk. Copenhagen: Hernov 1978–80.
Mellem jyske modstandsfolk 1941–1945 (Between Jutlandic Members of the Resistance Movement). Copenhagen: Zac 1980.
Sprogrenseren H.C. Ørsted (The Language Cleaner Hans Christian Ørsted). Volume 1–2. Aarhus: Nordisk Institut, Aarhus University 1981.
50 danske runeindskrifter (50 Danish runic Inscriptions). Aarhus: Nordisk Institut, Aarhus University 1981.
Syntaksen i Holbergs epistler (The Syntax in the Epistles of Holberg). Aarhus: Nordisk Institut, Aarhus University 1983.
Danske runeindskrifter: et udvalg med kommentarer (Danish runic Inscriptions: A Selection with Comments). Copenhagen: Hernov 1983.
Norske indslag i nydansk (Nordic Features in modern Danish). Volume 1: Dansk over for norsk sprog ca. 1700–1950. Volume 2: Liste over låneord fra norsk ca. 1700–1950. Aarhus: Nordisk Institut, Aarhus University 1983–84.
Fra runesprog til nudansk: studier og kommenterede tekster (From runic Language to modern Danish: Studies and commented Texts). Aarhus: Nordisk Institut, Aarhus University 1984.
(in collaboration with Hallfrid Christiansen) Norsk-dansk ordbog (Norwegian-Danish Dictionary). Gyldendal 1971.
(in collaboration with Sven Brüel) Gyldendals fremmedordbog (Gyldendals Dictionary of foreign Words). Copenhagen: Gyldendal, 9th edition 1983.