Probablebound to = certain: "They are bound to succeed!"
sure to = certain: "He is sure to win the championship."
likely to = probable: "We are likely to win the contract."
definite = sure: "He's a definite frontrunner for the job!"
probable: "It's probable that we will be on holiday around then."
likely: "An election is likely next year."
will definitely happen: "There will definitely be a storm later."
will probably happen: "They will probably take on more staff."
Possiblemay: "We may be able to help you."
might: "There might be a holiday next month – I'm not sure."
could: "There could be a bug in the system."
… is possible: "Do you think he will resign?" "Yes, that's possible."
… is unlikely: "It's unlikely that she will move."
will possibly: "She'll possibly tell us tomorrow."
probably won't: "They probably won't hear until next week."
definitely won't: "I definitely won't go to the party."
… is highly unlikely: "It's highly unlikely that the company will expand."
Note: Be careful of the word order.
"Definitely" and "probably" come after "will" (in positive sentences) and before "won't" in negative sentences.
VariationsYou can add words to alter the strength of probability:
highly likely / unlikely (= very likely / unlikely)
quite likely / probable / possible (= more likely, probable or possible)
could possibly / probably
most definitely won't (= even more unlikely)