The teacher reading this passage writes "s/v" in the margin in reference to sentence (12). what is the teacher trying to tell the writer?

The teacher is trying to say that there is a subject-verb error

Explanation:

When checking the writing in a piece of work, there are specific symbols to put little notes where the errors happen, the abbreviation S/V is used when there is a subject-verb agreement mistake, this is when it is used the incorrect verb form with any of the persons in the speech, for example, "The day is beautiful" instead of the correct form "The day is beautiful"

d. because santiago thought any sharks would assume the marlin was part of the boat