长句是合同中较为常见的情况，遇到长句不要慌，找出主谓宾等句子结构，拆分起来句子就没那么长了。以这句话为例，这句话很长，且包含了多个子句和条件短语，我们可以逐步分析。

分析：

主句:

主语：Buyer

谓语：may purchase

宾语：such quantity

状语：from the other manufacturer

状语：whereupon Buyer's commitment under this Agreement shall be reduced by the quantity so purchased.

条件句:

If Buyer furnishes proof to Seller...

主要条件：这是一个复杂的条件句，含有多个部分，包含以下几个子条件：

a. that Buyer can purchase from a manufacturer in any contract year any of the aforesaid materials produced within the United States

b. of the same quality, upon similar terms and conditions

c. in approximately the same quantity as the then undelivered quantity hereunder during such contract year

d. and at a lower price than is then in effect under this Agreement

结果：then if Seller shall not reduce the price hereunder to such a lower price for the aforesaid quantity, Buyer may purchase such quantity from the other manufacturer.

拆分翻译如下：

If Buyer furnishes proof to Seller:

如果买方向卖方提供证明，

that Buyer can purchase... at a lower price than is then in effect under this Agreement,

买方能以低于本协议当前有效的价格购买……，

of the same quality, upon similar terms and conditions,

具有相同的质量，基于相似的条款和条件，

in approximately the same quantity as the then undelivered quantity hereunder during such contract year,

数量大致与该合同年度内尚未交付的数量相同，

then if Seller shall not reduce the price hereunder to such a lower price for the aforesaid quantity,

那么，如果卖方不将本协议下的价格降低至对于上述数量的更低价格，

Buyer may purchase such quantity from the other manufacturer,

买方可以从其他制造商处购买此等数量，

whereupon Buyer's commitment under this Agreement shall be reduced by the quantity so purchased.

随后，买方在本协议下的承诺将减少所购买的数量。

整句话可译为：如果买方向卖方提供证据，证明买方能够在任何合同年度内，以更低的价格购买在美国生产的上述材料，且这些材料具有相同的质量、相似的条款和条件，数量大致相同，那么，如果卖方不降低本协议项下的价格至更低价格，买方可以从另一制造商那里购买该等数量，随后，买方在本协议项下的承诺将减少所购买的数量。