This means you are very sure about something. You think it must be true. It is like saying 'it must be...' or 'surely...' in English.
Verb [dictionary form] + べしPrimary
い-Adjective + べし
な-Adjective + である + べし
Noun + である + べし
This means you are very sure about something. You think it must be true. It is like saying 'it must be...' or 'surely...' in English.
かの文豪が残した未完の原稿、その結末はかくあるべし。
Regarding the unfinished manuscript left by that great literary figure, its conclusion must surely be this.
この空模様、今夜は嵐になるべし。
Judging by the look of the sky, there will surely be a storm tonight.
彼が最後に見た景色は、さぞ美しかるべかりし故郷の山々であったろう。
The last sight he saw was likely the mountains of his hometown, which must have been so beautiful.
彼の計画は緻密であり、成功は疑う余地もなかるべし、だが一抹の不安が残る。
His plan is meticulous, so its success must be beyond doubt, yet a trace of anxiety remains.
この城を築きしは、かの有名な武将なるべし。
The one who built this castle must be that famous warlord.
べし sounds very old-fashioned. It's like speaking in a formal, serious way. It makes your guess sound very strong and sure, not just a random thought.
This is different from other forms like 〜む or 〜ん. Those just guess. This form means you are very sure. It is also different from 〜べし when it means "you must do something." This form means "it must be true." The situation tells you which meaning to use.
Don't use this when you mean "must not be." For that, use "〜まじ." This form is not for saying "must not do." That is "〜べからず."
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