Should I use "is" or "are" in the following case?
What I'll never forget, though, is the first days I started noticing her.
What I'll never forget, though, are the first days I started noticing her.
Here, the main topic is those "first days," so I figured I should use "are." However, here I'm also talking about "What" so someone said I should use "is."
What's the correct option?
If you said "The thing I'll never forget," you may certainly say "is" because the subject is clearly singular.
"The thing I'll never forget, however, is the first days that I noticed her" is not a strictly logical sentence because it is equating a singular to a plural. But language and grammar are not strictly logical. What is intended is something like "The thing I shall never forget, however, is my response during the first days that I noticed her." The grammar respects the ellipsis.
"What I shall never forget" does not specify whether it is singular or plural, but the complement is plural. That certainly permits "are" as the verb. However, it would be natural to treat the verb as a singular if, in fact, the intent was to speak not of the days themselves, but your reaction during those first days.
In speech or informal writing, "is" would be so common that it would be pedantic to call it wrong. In formal writing, "are" is probably preferable if you literally mean that you remember the days, but if you really mean something else, avoid a sloppy ellipsis.
"What I shall never forget, however, were my feelings during those days when I first noticed her."
"What I shall never forget, however, was my reaction during those days when I first noticed her."
In these examples, "what I shall never forget" takes on the number of the complement and so determines the number of the verb.
We use ellipsis a lot in speech, and grammar accommodates to it. In formal writing, where there is no tone of voice, gestures, or opportunity for back and forth, avoid ellipsis because it is frequently unclear.
EDIT: In formal writing, I'd dispense with the verbosity of speech and just write: "I shall never forget __ during those first days that I noticed her." That eliminates the problem altogether.
