Complex sentence: Since the top quark is extremely short-lived, only the charm and bottom quarks are of significant interest to HQET, of which only the former/latter has mass sufficiently high that the effective theory can be applied without major (perturbative?) corrections.
Split into simpler sentences: Since the top quark is extremely short-lived, only the charm and bottom quarks are of significant interest to HQET. Only the former/latter has mass sufficiently high that the effective theory can be applied without major (perturbative?) corrections.