I shall provide you with information regarding the usage of these two words. Firstly, you should know that ''likely'' and ''probably'' have pretty much the same meaning. However, there is a subtle difference between them:
* ''Probably'' is an adverb, so it describes the manner in which a verb is performed. The confusion over these two words stems mostly from people's use of 'likely' to mean 'probably'. ''Likely'' can be an adverb as well, but its distinguishing feature is that it often acts as an adjective, a function 'probably' cannot perform. When it acts as an adjective, 'likely' must describe something.
E.g.: ''The likely winner appeared at the last minute.''
E.g.: ''You are likely to eat all of the cake if I leave you here alone with it.''
E.g.: ''It is likely that there is a better way to trap a mouse.''
Now here comes the confusing part. When 'likely' acts as an adverb, it needs a qualifying word in front of it, such as 'most', 'more than', 'very'or as part of the phrase 'more likely than not' (Hint: Do not use it as an adverb on its own):
E.g.: ''We'll most likely leave in the morning.''
E.g.: ''The book quite/very likely has more than one author.''
E.g.: ''More than likely, the cause of her illness is stress.''
E.g.: ''More likely than not they would kill him if they found out who he really was.''
** Many grammarians hold that it would be incorrect to write these sentences without the qualifiers. Instead, 'probably' is required.
E.g.: ''We'll probably leave in the morning.''
E.g.: ''The book probably has more than one author.''
(Hint: If you want to include the qualifiers, make sure you use 'likely' and not 'probably'. The latter can't take a qualifier!)
Hope that helps!!! (You are likely to have understood it :)
Should you have any further doubts, do not hesitate to contact me. I shall be pleased to help!