Grammar Info
N2 Lesson 8: 5/22

恐(おそ)らく
probably, perhaps, (most) likely, (I) fear that

Structure
おそらく + Phrase
Details
Register Formal
使用域 硬い
Information
恐(おそ)らく is an adverb which originally stems from the conjugation of the yodan verb, 恐(おそ)る ‘to fear’. In most cases, 恐(おそ)らく is simply used as a formal way to say ‘probably’, or ‘perhaps’, and is usually partnered with other structures of conjecture such as だろう, かもしれない, etc.
As 恐(おそ)らく behaves adverbially, it will most often appear at the beginning of sentences before a statement of conjecture is made.
  • 妻(つま)は今夜(こんや)会社(かいしゃ)の人(ひと)たちと飲(の)み会(かい)に行(い)くと言(い)っていたので、おそらく11時(じ)ごろぐらいまでは帰(かえ)ってこないと思(おも)います。
    My wife said she was going to a drinking party tonight with some people from work, so she probably won't be home until around 11:00 or so.
  • あいつの不自然(ふしぜん)な行動(こうどう)からして、おそらくあいつが犯人(はんにん)だろう!
    Judging by that guy's unnatural behavior, he's probably the culprit!
  • おそらく明日(あした)は雨(あめ)かもしれないので、明日(あひた)予定(よてい)していたピクニックは中止(ちゅうし)しましょう。
    Perhaps it may rain tomorrow, so let's cancel the picnic we had planned for tomorrow.
Although 恐(おそ)らく can, and often does, carry the meaning of ‘I fear that (A)’, it isn’t necessarily always used in relation to negative things. In the case of this particular grammar pattern, the main nuance that ‘I fear that (A)’ conveys is modesty on behalf of the speaker in that they fear their assumption may be right, given the available information.
Fun-fact - yodan, or 四段活用(よだんかつよう) verbs are a type of classical-Japanese verb that have primarily been replaced by godan (う-Verbs) in modern day Japanese.
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おそらく彼(かれ)ここにいないだろう。
He most likely isn't here.
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おそらく明日(あした)雨(あめ)降(ふ)る
Perhaps it will rain tomorrow.
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彼(かれ)だったら、おそらくこんなことできっこないでしょう。
If it’s him, he most likely won’t be able to do it.
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Books
Tobira
Page 261
Tobira (Gateway to Advanced Japanese)
Page 218-3
新完全マスター文法、N2
Page 92-5