Explain the rules governing the passing of risk in a sale of goods contract

Rules governing the passing of risk in sale of goods contract.        

• In an unconditional contract for sale of specific goods in a deliverable state property passes at time of contract.
• In a contract of sale of goods that are in a deliverable state but the seller has to do something to determine the price; property does not pass until that thing is done and price is determined.
• In a contract of sale of goods on sale or return basis property passes when goods are accepted by buyer.
• In a contract of sale of future or unascertained goods, property passes when the goods become ascertained.