首先，需要搞清楚这两个词中的“constructive”是什么意思。

“constructive”除了常见的积极的、建设性的意思之外，还有推定的、推断的、法律归责的意思，指虽不一定是事实，但应承担法律后果。法庭出于公平考虑，常作此推定性认定。

接下来这两个词就比较好理解了：

“constructive breach”预期违约。

指有合同履行义务之一方当事人，在合同履行日到来之前，以某种行为或声明表示其将不履行合同义务。它构成预期违约。Constructive breach occurs when the party who is bound to perform withdraws from performance by some act or declares before the time comes that he will not perform.

“constructive contract”推定的合同；准合同

不是依据当事人的意思表示，而是为避免不公正从而按照法律成立的合同。又称准合同〔quasi contract〕或法律上的默示合同〔implied contract in law〕，以区别于事实上的默示合同〔contract implied in fact〕，在后一种合同中，当事人是用行为而不是语言文字为订立合同的意思表示，是真正的合同；而法律上的默示合同是法律为公正起见规定的一种义务，并不考虑当事人是否用言词或行为作出同意的表示。A constructive contract is a legal obligation between the parties involved in a dispute in the absence of a prior obligation between the parties. This prevents unjust enrichment. While a normal contract has different elements and requires the parties to be in agreement, a constructive contract has no such requirements and is usually invoked for restitution. A constructive contract is also known as a quasi contract.

以下为相关双语示例，供参考：

You can commit a constructive breach of contract if you don’t pay your mortgage as outlined in the mortgage contract but you can repair the breach by paying your past due installments. You will still be responsible for late charges but you cannot be charged extra in the future.如果您没有按照抵押合同中的规定支付抵押贷款，您构成预期违约，但您可以通过支付逾期分期付款来纠正违约行为。您将仍需负责缴纳滞纳金，但未来并不会向您收取额外费用。

Constructive contract outline the obligation of one party to another when the latter is in possession of the original party's property. These parties may not necessarily have had a prior agreement with one another.

准合同概述的是当一方成为原始当事方财产的新的持有人时，另一方对其的义务。这些当事方不一定事先与对方达成了协议。