Final answer:
To make the sentence negative, insert 'not' after the auxiliary verb, making it 'His memory is not very sharp.' This expresses that the person's memory isn't sharp, considering conditions like anterograde amnesia and transience which affect memory retention.
Step-by-step explanation:
To make the sentence "His memory is very sharp" negative, you need to add a negation word like 'not' after the auxiliary verb 'is.' The resulting sentence would be "His memory is not very sharp." This change alters the meaning of the sentence to convey that the person's memory isn't sharp, directly contradicting the original statement. When dealing with subjects related to memory, it's important to consider context, such as cases of anterograde amnesia, where a person can't form new memories, or transience, where memories can fade over time, as evidenced in the inability to recall the plot of a book like 'To Kill a Mockingbird' after many years.