Talking about probability in English

There are many ways of saying that something will probably or possibly happen.

Probable

bound 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."

Possible

may: "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.

Variations

You 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)

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