“Zayd is writing” is about a specified individual, hence singular. “Some humans are writing” refers to some instances of a universal subject, hence a particular-quantified proposition.
Singular: a determinate individual — “This teacher is present.”
Particular-quantified: some members of a class — “Some teachers are present.”
Neither ranges over all members of a class, but they differ in kind. Singularity fixes an individual; particular quantification specifies a portion without identifying the individuals.