I know that these stems for the future and conditional tenses are found in Spanish, and also about the whole inf + habere ad origin of the Romance future/conditional tenses. But I don't understand how French came to acquire ser and ir? French merged esse and ester into être, whereas Spanish merged esse and (e)sedere into ser (and kept estar separate). I don't get how French managed to pick up ser yet keep esse (to merge into ester) at the same time. I also note that ir is the equivalent of "aller" in Spanish, but I wonder how French picked the ir- stem up?