べく

Expresses purpose: "in order to do," "with the aim of doing." The preceding clause states a goal, and the following clause describes a deliberate action taken to achieve it.

A formal, literary pattern used in written Japanese such as news reports, speeches, business writing and narratives. It stresses a strong, deliberate intention behind the action. It is often used when describing a person's or group's efforts toward a goal, where everyday speech would use ために.

• んがためN1「んがため」 also expresses purpose in formal written style and can often be swapped with 「べく」, but it carries a sense of urgency — the resolve to achieve the goal no matter the cost. It attaches to the ない-form with ない removed and ん added; with する it becomes 「せんがため」, and 「んがための＋名詞」 is also possible.
• ためにN3Besides purpose, 「ために」 can also express cause; when expressing purpose it can additionally attach to 「名詞＋の」, is neutral in register, and works in both speech and writing. 「べく」 expresses only purpose, attaches only to verbs, and is more formal and literary; 「べく」 used for purpose can generally be replaced with 「ために」.
• べくしてN1「べくして」 must be used in the pattern 「動詞辞書形＋べくして＋同一動詞た形」, with the same verb in dictionary form before べくして and in た form after; it looks back to evaluate that the result was bound to happen, or that something was rightfully gained through one's own ability. It does not express purpose and cannot be swapped with 「べく」.
• べくもないN1「べくもない」 likewise attaches to the dictionary form of verbs and likewise belongs to written style, but it does not express purpose; it means 'utterly impossible, no way to ...' and is mostly used with a small number of verbs such as 望む, 知る, 疑う, and 比べる. It cannot be swapped with 「べく」.
• にはN3When 「には」 expresses purpose, what follows is usually a necessary condition or an evaluation (e.g., 「必要だ」), whereas 「べく」 is usually followed by the action taken for that purpose or by a call to action, with a formal, literary tone; the two are generally not interchangeable. 「には」 can also attach to a noun to mean 'for ...'.
• もさることながらN1「もさることながら」 means 'A goes without saying, and B even more (also) ...', with the emphasis on the second item, so it differs in meaning from purpose-expressing 「べく」 and the two cannot be swapped. It attaches to nouns while 「べく」 attaches to the dictionary form of verbs, and both lean toward written style.

Unlike neutral ために (which also attaches to nouns) and the even more archaic, emphatic んがため, べく is a formal purpose marker used after verbs; には marks a purpose followed by an evaluative statement ("to do X, Y is needed"). べくして ("as was bound to happen") and べくもない ("there is no way to") share the form but do not express purpose, and もさることながら means "not only X but also Y."