作者：範文祥，著有暢銷書《英文合同閱讀與分析技巧》。
英文合同使用頻率最高的兩個情態助動詞，非may和shall莫屬。
通常情況下，若是賦予（剝奪）或增加（減損）權利（right）、特權（privilege）或權力（power），用may或may not。若是施加義務，或承擔責任，則用shall。若是禁止做什麼，則用shall not或者shall後直接加表示禁止的實義動詞。
May和shall的用法，正好契合了英文合同的主要內容，即合同各方的權利及其實現、義務及其履行和風險/責任及其分擔，所以被廣泛使用。
下面以《聯合國國際貨物銷售合同公約》（United Nations Convention on Contracts for the International Sale ofGoods，CISG）和《防止船舶汙染國際公約》（International Convention for the Prevention of Pollution from Ships）兩個公約的措辭為例分別予以說明。
《聯合國國際貨物銷售合同公約》是目前影響範圍最為廣泛的國際法律文件。公約總共101條，但卻用了77個may。
例1 CISG公約第16條第1款
Until a contract is concluded an offer may be revoked if the revocation reaches the offeree before he has dispatched an acceptance. 在合同訂立之前，要約方有權撤銷要約
賦予權利，用may。
例2 CISG公約第29條第1款和第2款
(1) A contract may be modified or terminated by the mere agreement of the parties.合同締約方有權協議變更或終止合同
賦予權利，用may。
(2) A contract in writing which contains a provision requiring any modification or termination by agreement to be in writing may not be otherwise modified or terminated by agreement. 規定協議變更或終止合同必須採取書面形式的書面合同，不得以任何其它方式協議變更或終止
公約第29條第2款是對第29條第1款所述權利的限制/減損，故用may not。
例3 CISG公約第47條第1款和第2款
(1) The buyer may fix an additional period of time of reasonable length for performance by the seller of his obligations.
賦予買方權利，用may。
(2) Unless the buyer has received notice from the seller that he will not perform within the period so fixed, the buyer may not, during that period, resort to any remedy for breach of contract. However, the buyer is not deprived thereby of any right he may have to claim damages for delay in performance.
第一個may not限制/減損買方的救濟權利（remedy），而第二個may又是賦予損害賠償權，表達的是「may have right to claim damages for delay in performance」。
但在《防止船舶汙染國際公約》（International Convention for the Prevention of Pollution from Ships）中，整個公約全文使用了98個shall。
例4 In respect of the Antarctic area, any discharge into the sea of oil or oily mixtures from any ship shall be prohibited.
shall加禁止的實義動詞，表述禁止。
例5 Oil tankers of 150 gross tonnage and above shall be equipped with an oil discharge monitoring and control system approved by the Administration和The Oil Record Book Part I, shall be kept in such a place as to be readily available for inspection at all reasonable times and, except in the case of unmanned ships under tow, shall be kept on board the ship.
shall表示義務。
May和shall的上述用法，主要用在法律文件中，與普通人日常英文中的may和shall的用法有別。近年來，may和shall的這些法律用法，廣受詬病，尤其是shall，被指語義模糊，既可以表示must，也可以表示may或will，國際上興起的採用簡明英文（Plain English）草擬法律文件的熱潮更是使shall成為千夫所指。
美國在七十年代就開始在政府行政部門推行「非專業律師術語」（Layman’s Terms），尼克森、卡特和柯林頓政府都先後對此提出要求，要求法律法規必須儘可能簡單明了(be as simple and clear as possible)；美國證監會(SEC)還特意出版了一個小冊子(A Plain English Handbook)，指導大家怎麼用簡單的語言進行披露；美國歐巴馬政府甚至籤署了（Plain Writing Act），目的在於確保政令清楚，便於普通人理解和使用；而澳大利亞也在其立法中使用must代替shall。
再看《聯合國國際貨物銷售合同公約》，對於買賣雙方的義務和責任部分，全都用must代替了shall。比如
例6 CISG公約第41條
The seller must deliver goods which are free from any right or claim of a third party, unless the buyer agreed to take the goods subject to that right or claim. 賣方所交付的貨物，必須是第三方不能提出任何權利或要求的貨物，除非買方同意在這種權利或要求的條件下，收取貨物
例7 CISG公約第53條
The buyer must pay the price for the goods and take delivery of them as required by the contract and this Convention.買方必須按照合同和本公約規定支付貨物價款和收取貨物
例8 CISG公約第77條
A party who relies on a breach of contract must take such measures as are reasonable in the circumstances to mitigate the loss, including loss of profit, resulting from the breach. 聲稱另一方違反合同的一方，必須按情況採取合理措施，減輕由於該另一方違反合同而引起的損失，包括利潤方面的損失
與此形成鮮明對比的是，國際標準組織（ISO）和各種行業協會制定的標準合同模板卻仍堅持使用shall，而非must。比如國際諮詢工程師聯合會（FIDIC）制定的合同模板中，僅《設計採購施工合同》一個合同模板就使用了600多個shall，卻未使用一個must。
比如
1.4 The Contract shall come into full force and effect on the date stated in the Contract Agreement. The costs of stamp duties and similar charges (if any) imposed by law in connection with entry into the Contract shall be borne by the Employer.合同自合同協議書規定的日期起全面實施和生效。為籤訂合同協議書，依法徵收的印花稅和類似的費用（如果有）應由僱主承擔
1.12 The Contractor shall not be required to disclose, to the Employer, any information which the Contractor described in the Tender as being confidential. The Contractor shall disclose any other information which the Employer may reasonably require in order to verify the Contractor’s compliance with the Contract. 不得要求承包商向僱主透漏他在投標書中稱為是秘密的任何信息.對僱主為了證實承包商遵守合同的情況，正當需要的其他信息，承包商應當透漏
2.5 The notice shall be given as soon as practicable after the Employer became aware of the event or circumstances giving rise to the claim. A notice relating to any extension of the Defects Notification Period shall be given before the expiry of such period. 通知應在僱主了解引起索賠的事件或情況後儘快發出.關於缺陷通知缺陷任何延長的通知,應在該期限到期前發出
綜上，may和shall在各類英文合同中仍將長期存在，在草擬和翻譯時，一定要特別注意。
本公眾號旨在分享英文合同閱讀、分析、談判、翻譯、草擬實戰經驗，無意對各種經院式理論進行站隊辯論，更無意進行說教，惟願與同道中人一同切磋，一路同行。雖知蚍蜉撼樹，蟪蛄春秋，但亦知聚沙成塔，薪火相傳。
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