英语合同中不同的情态动词约束的权利和义务也不同，选词的时候要谨慎。

shall约定当事人的义务，表示强制性要求履行责任或义务；

例：The Seller shall present the following documents required for negotiation to the banks. (卖方必须将下列单据提交银行附议)

should表示“应该”，只表示一般的义务或者道义上的义务； 

Should the verification conclusion contradict the conditions of an auction target stated in an auction contract, the auctioneer has the right to demand a change or rescind the contract. (如果鉴定结论与委托拍卖合同载明的拍卖标的状况不相符，拍卖人有权要求变更或者解除合同。)

may旨在约定当事人的权利，没有任何义务的含义，不带有强制性，有时表示“允许”或“许可”；

例：The buyers may, within 15 days after arrival of the goods at the destination, lodge a claim against the sellers for short weight being supported by Inspection Certificate issued by a reputable public surveyor. (货物抵达目的港15天内，买方可以凭有信誉的公共检验员出示的检验证明向卖方提出缺重赔偿。)