Regarding the example, "Mi madre conduce rápido cuando sale tarde para la oficina," can you replace "sale" with "saliendo," as you could in English? If I recall correctly, the present participle can still be used as an adjective, so it seems that this would work as well (saliendo would still be describing the mother).
1 Answer
No. You must use "estar" if you want to use the gerund:
Mi madre conduce rápido cuando está saliendo tarde para la oficina
You have the option "al salir"
Mi madre conduce rápido al salir tarde para la oficina
Or
Mi madre conduce rápido cuando sale tarde para la oficina
This last one is the most natural to me, because patterns are better represented by present simple --presente de indicativo-- in Spanish too.
