Complex sentence: He took second place at Viña del Mar 1945, with 10.5/13, behind Guimard, then won Mar del Plata 1945 with 11/15, ahead of Ståhlberg, and repeated at Mar del Plata 1946 with 16/18, ahead of Guimard and Ståhlberg.
Split into simpler sentences: He took second place at Viña del Mar 1945, with 10.5/13, behind Guimard. Then he won Mar del Plata 1945 with 11/15 ahead of Ståhlberg, and repeated at Mar del Plata 1946 with 16/18, ahead of Guimard and Ståhlberg.