Mordechai Gebirtig (1877 – 1942) was the most popular and beloved Yiddish songwriter of the 20th Century.  His songs express the heart and soul of each person and at the same time reflect the feelings and striving of the Jewish people as a whole.  So popular were his songs that many were mistaken for folksongs.  Among them are Reyzele (Reisl) — about  young love;  Yosl Ber — about joining the army, and   Undzer Shtetl Brent (Our Town is Burning)  They are by…