恐らく + [sentence] + だろう / でしょう 恐らく + [sentence] + と思う
恐らく is an adverb meaning "probably", "most likely". It goes at the start of the sentence or before the predicate, and the sentence nearly always ends in a guess: だろう, でしょう, とおもう思う or のではないか. It is often written in kana, おそらく.
Both mean "probably". たぶん is the everyday word and fits any conversation. 恐らく sounds weighed and a little formal, so it belongs to news, reports, presentations and serious talk, and it suggests you are fairly sure.