What is a mixed condition?

Mixed condition is when the fulfilment of the condition depends partly upon the will of a party to the contract and partly upon chance and/or will of a third person. 

There are two doctrines that you need to know when talking about mixed conditions.

Doctrine of Constructive Fulfillment of Suspensive Condition. This doctrine requires that the condition be suspensive, that the debtor actually prevents the fulfillment of the condition, and the debtor acts voluntarily. If these three requisites are present, then Article 1186 of the Civil Code applies, to wit:

ART. 1186. The condition shall be deemed fulfilled when the obligor voluntarily prevents its fulfillment. (Emphasis applied)

If the condition depends partly on the will of the debtor and partly on chance or the will of the third person, and the part that depends partly on chance or the will of the third person was already fulfilled but the debtor intentionally prevents the fulfillment of the part of the condition that depends on him, the doctrine of constructive fulfillment of suspensive condition is applied.

Doctrine of Constructive Fulfillment of Mixed Condition. If the debtor did all in his power to comply with his obligations and the condition has not been fulfilled because the third person did not comply and is in no way be compelled to fulfill the condition, the condition is deemed satisfied. This is known as the rule on constructive fulfillment of mixed condition.