Why “can” not “could” in “She was one of those people who can. . . .”?

The following sentence is from George Orwell’s Nineteen Eighty-Four, occurring in chapter 5 of part 2:

She was one of those people who can go to sleep at any hour and in any position.

Why is who can grammatically correct when used with was? Shouldn’t it be who could?

The description is taken from a novel; in that context, it is usual to refer to people using the past tense.

Who could relates not to the character just mentioned, but to the generic referent one of those people, whose characteristics can be described both by who can and who could.

"Can" means that that someone is able to do XYZ. "Could" is the past tense of "can," but also can be used to express the conditional: "she could have..."

She was one of those people who can to go to sleep at any hour and in any position.

"One of those people" who can do XYZ are still those people, even though she was formerly one of them. I think that's why the original sentence is acceptable, although "could" is equally viable.

This is a delicate issue of finite vs non-finite verbs. What are they???

If you are familiar with object-oriented grammar, you would immediately hit a lit lite-bulb when I ask you to compare classes versus objects.

If you are a statistician, your lit lite-bulb would be category-dimensions vs values.

If you are a US naval officer, your lit lite-bulb would be Nimitz-class vs The George Washington.

There are two (i.e. I know of two) modes of linguistics that is used for modular displacement of predicates.

• non-finite verbs
• subjunctives (general terminology, not the English language specific term)

Non-finite verbs are classes or categories of action, and therefore their actions are not bound to a specific instance.

Example of a non-finite verbs, or non-finite verb in participle form:

• I love painting houses.
• I love painted houses.

Example of finite verb clauses:

• I am painting this house.
• I painted a house last summer.

The general perspective you need to bear is there are times we need to speak of general classes of action, while at other times we need to speak of specific actions.

Examples uses of a time-independent/instance-independent modular clause (i.e. non-finite verbal clauses/phrases):

• Past participle: White-tailed deer are cute.
• Past participle: Painted doors are beautiful.
• Present participle: I am {having breakfast}.
• Verbal-noun gerund: This is a beautiful painting.
• Gerund: I love {having breakfast}. I have been {having breakfast} everyday sinec I was born. After I die, I will no longer be {having breakfast}.
• Subjunctive: I know of people who {could sleep at anytime of the day}.
• Imperative: Do not cross the road without a green walking light.
• Infinitive: I had advised her to {speak to you}.
• To-less infinitive: : I had expected that she {need not go to work}.
• +To-less infinitive: I had enjoyed seeing {people laugh at themselves}. Seeing {people laugh at themselves} always brightens up my life. Ten years from now I will still enjoy seeing {people laugh at themselves}.
• +gerund: I had enjoyed {seeing her {laugh at herself}}. {Seeing her {laugh at herself} always brightens up my life. Ten years from now I will still enjoy {seeing her {laugh at herself}}.
• gerund: I had enjoyed {people laughing at themselves}. {People laughing at themselves} always brightens up my life. Ten years from I would still enjoy {people laughing at themselves}.

Addendum

+A change has been made to one of the examples, due to F.E.'s advisory.