Hola,
I thought assumption about the past (must have + verb) was expressed by means of DEBE (Deber in El Presented) + Infinitivo Compuesto, however the following examples refute it:
Los chicos han debido de beber bastante porque huelen a alcohol.
They boys must have drunk quite a lot as they smell of alcohol.
El tren debió de llegar sobre las cuatro y media.
The train must have arrived at about four thirty.
Could you please clarify this point, because I'm feeling a bit confused