Klajn was born in Belgrade to , a physician and psychoanalyst, whose
Croatian-Jewish family had lived in
Vukovar for generations, and , who was a pianist from a Serbian merchant family. During World War II, they were separated because his father went into hiding to avoid Nazi persecution, which claimed the lives of several of his sisters. They rejoined by the end of the war, and his father was later a theatre director. His mother was a musicologist who travelled to the US in the 1950s, and introduced Ivan to
Ljubica Marić who would teach him music theory. His mother spoke French and his father spoke German, and he learned English from an interest in jazz. Klajn served his military service in Zagreb where he met his father's friend
Miroslav Krleža. He completed the studies of the
Italian language and literature at the
University of Belgrade Faculty of Philology. After graduation, he started working at the same faculty, becoming a regular professor of the Italian language and the comparative grammar of Romance languages. Beside Romance Studies, his interest also lies in the language
morphology, linguistic consultancy and the standardization of the modern
Serbian language. His major works are the
Rečnik jezičkih nedoumica ('The dictionary of linguistic dilemmas'),
Tvorba reči u savremenom srpskom jeziku ('Morphology in modern Serbian language', 2 volumes) and
Italijansko-srpski rečnik ('Italian-Serbian dictionary'). In 2017, he signed the
Declaration on the Common Language of the
Croats,
Serbs,
Bosniaks and
Montenegrins. Klajn died from
COVID-19 on 31 March 2021, in
Belgrade at the age of 84. ==Books and articles==