Possible, probable (adj) and likely (adj & adv) all denote "likelihood" in meaning.
However, there are some subtleties between them.
First of all, "possible" emphases the likelihood in an objective manner, but it always implies a sense of "the likelihood is minimal". For example, I would say that while failure is possible, everything points towards success.
Next, "probable", according to its etymology, means verifiable, and thereby extends to mean "substantiated, reasonable, and credible". In contrast to "possible", it may well indicate a strong sense of likelihood.
For example: (1) It's possible, thought not probable, that he would accept the offer. (2) The probable cause of his death has been diagnosed as avian flu.
Finally, "likely", on the surface of it, is similar to "probable". They are sometimes interchangeable with not much difference in meaning.
For example: It is likely (probable) that we shall meet with some opposition.
The difference between "likely" and "probable", if you like, will be that the former sets great store by the judement that is arrived at from some superficial signs, whereas the latter is based on the truth that is revealed after considering the plus side and negative side of a situation.
For example, (1) The likely outcome of the contest varies from moment to moment(Either A or B will win as both of them are equal in strength and skills. The outcome is unclear). (2) On the probable outcome of the contest, opinions of the audience are unanimous(Obviously, A is stronger than B). Hope that helps a bit.
(Not a teacher)
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