恐らく is an adverb that indicates the speaker's high-probability guess. It often pairs with verbs like だろう, でしょう, と思う, or に違いない to strengthen the conjecture. While it can be used with a plain statement, it's more natural to combine it with expressions of uncertainty to match its hedging nature.

In speech, it's common to say 恐らく～でしょう or 恐らく～だろう. In writing, you might see it with ～と思われる or ～に違いない. Avoid using it in very formal business contexts where a more objective tone is preferred.

Note that 恐らく is not used in negative sentences to mean 'probably not' as strongly; instead, たぶん～ない is more common, though 恐らく～ない is possible but sounds more formal.