[1][2] He graduated from secondary school (Gimnazjum no 4) in Kraków in 1917, then studied law (1917–1919) and history (1919–1922) in the Jagiellonian University, among others under the direction of Władysław Konopczyński and Stanisław Kot.

In 1922 he was appointed assistant in the Seminar of History of the Jagiellonian University, in 1923 he defended his doctor thesis Polska w dobie wielkiej wojny północnej 1704-1709" [Poland in the Time of the Great Northern War 1704-1709] elaborated under the supervision of Stanisław Kot.

He was a pioneer (after his predecessor Kazimierz Jarochowski) of scholarly research of the history of Poland in time of the Great Northern War during the reign of Augustus II the Strong.

He published a series of monographs: Polska w dobie wielkiej wojny północnej 1704-1709 [Poland in the Time of the Great Northern War 1704-1709] (1925) continued in Polska a sprawa wschodnia 1709-1714 [Poland and the Eastern Question 1709-1714] (1926) and Geneza Konfederacji Tarnogrodzkiej [The Origins of the Tarnogród Confederation] published in "Kwartalnik Historyczny" (1928), and a monograph of king Stanisław Leszczyński (1948, republished in 1959 and 1984).

Feldman was the author of the statement, that the period of the Wettin dynasty in Poland was featured not only by a decline of the country, but it was also the beginning of progressive thought and inner rebirth of the nation.

In his work Sprawa polska w roku 1848 [The Polish Question in 1848] (1933) he presented an outline of Prussian and Russian policy towards the situation on lands of partitioned Poland during the Revolutions of 1848.