Se usan estos verbos para describir conclusiones o suposiciones.

verbo ejemplos
must You must be insane to jump off a moving train.
should / ought to He should be here in five minutes. / She ought to have bought the cabbage.
can This can only be Pete driving like a maniac in our backyard.
could They could be still at work or they could be in the bar next door.
may She may have left already, but I can't tell for sure.
might This might be the wrong way we are going.
will It is seven o'clock, he will be arriving at New York just now.

Al usar el indicativo se presentan los hechos como más bien probables, al usar el subjuntivo se presentan como más bien improbables. Con complementos circunstanciales de tipo maybe, possibly, it's likely, it is certain, I'm sure, if I'am right también se pueden expresar conclusiones o suposiciones.

verbo ejemplo
to assume I assume they are at the theatre.
to presume He presumed that she was at the hairdresser's
to guess She guessed we were at the bar next door.
maybe Maybe he went to the shop to get some milk.
possibly They possibly got lost on the way to here.
if I'm right She didn't go to school, if I'm right (if my feeling is right etc.)
if I'm not mistaken If I am not mistaken, I would say that he has broken a leg.