Why is "a couple of..." correct grammar, while "a few of..." often isn't?

Earlier today I started to type a message and I entered:

I can take a couple of hours...

After entering the text, I realized that I intended to express roughly three hours so I highlighted and replaced the word "couple" with "few".

The resulting statement, before further edits, was:

I can take a few of hours...

Being that I am an American and English is my primary language, I paused for a moment to consider this phrasing of the second statement. It is completely wrong and feels awkward but I have no clue why.

To further confound the problem "a few of" is not always improper wording. Consider "I will take a few of them." In this context, "few of" is correct and, simply, "few" is wrong?!

The similarities and nuances of the words "couple" and "few" have been frequently discussed, even here on English.SE. Even after reading a few questions relating to these words on this site, it has always been my working assumption that these words have the same general meaning, except a couple is a relative quantity of two while a few was a relative quantity of three or a few more (pun intended.)

What is the nuance difference between these two words, in light of the above context?

Solution 1:

There are two differences between "hours" and "them". "Hours" is an ordinary noun, and hours are usually indefinitely divisible. (An "hours" in the context of a "one hour appointment" is a discrete, countable noun, however.) "Them" is a pronoun, and stands for discrete, countable items. There's room for a third category in this discussion: ordinary nouns for discrete, countable items – "cookies", for example.

• A couple of hours connotes a time period of about two hours, maybe one-and-a-half to two-and-a-half hours.
• A few hours is more vague; its meaning depends on context.
• A couple of cookies is very concrete: it's two cookies.
• A few cookies is also vague (and in the case of cookies, probably prone to rationalization).
• A couple of them refers to two specific items; which items are implied by context.
• A "few of them" refers to a small, indeterminate number of specific items.

So why a few hours or a few cookies, but a few of them? That brings to mind a fourth possible category: specific cookies. Although "a few cookies" is correct and "a few of cookies" is wrong, "a few of the cookies" is correct.

So, what's the conclusion? I think "a few of" must refer to a small number of items from a specific collection of items.

Solution 2:

"Few" is a determiner, i.e. it directly modifies nouns- "few people, a few mishaps," etc. "Couple", however, is a noun, so you need to specify what it is a couple of. "A couple of my coworkers, a couple of them," etc. Interestingly, in my colloquial dialect, it's become common to say "a couple [noun]", dropping the "of". Hope this helps.