The usage of "the" with "least"

The main issue here is how to sort out the usage of "the" with "least". Sometimes it's clear but there are cases when I am not sure whether to use the article "the" or not.

Least with verb

• The man who actually won the car is the one who wanted it least. [No “the”.]
• The President seemed to be the one who knew least about the crisis. [No “the”.]

(The sample sentences are from a dictionary.)

But:

• I earn the least out of all of us. [Again from a dictionary.]
• Which subject do you like the least? [Found randomly on the Internet.]

I have come to a vague idea which can explain it to some extent. For example: I earn the least out of all of us. = I earn the least (amount of money) out of all of us. So, when “the” is used it adds an idea of substantivisation i.e. “the least” is a noun.

However, it seems like there are cases when my explanation doesn’t work. E.g., "Which subject do you like the least?" I don’t see how it can be interpreted as a noun. I think it should be "Which subject do you like least?"

Least with participle

• I'm not the least surprised that she's leaving. [Taken from a dictionary.]
• The reply will discuss the one you are least interested in. [Found randomly on the Internet.]

I think it should be: "The reply will discuss the one you are the least interested in."

I am not sure if my ideas are correct, I am just making them known to you so that you can understand the way I think in respect to the issue and shed some light on the issue.

Solution 1:

The word "least", perhaps from getting a lot of use, is flexible in the roles it fills as a part of speech. The American Heritage Dictionary lists it with three definitions, all with approximately the same meaning, but as an adjective, adverb, and noun.

So taking an example:

I earn [the] least out of all of us.

If the "the" is there, "least" acts as a noun, the direct object of what was earned (the smallest amount). If the "the" is omitted, "least" becomes an adverb describing how you earned. They both end up with very close meanings, no one would think very hard about it, and so either one sounds right.

A complicating factor is that "the least", as a phrase, has also come to have adverbial status. I speculate that started with prepositional phrases like "in the least bit" fulfilling an adverbial role, and then wearing down over time. As a result, "the least" usage tends to signify something is very small or insignificant in general. "Least" by itself can also act that way, but can also have a tendency to invite a comparison. In a sentence like

The reply will discuss the one you are [the] least interested in.

it doesn't matter all that much. But sometimes it does matter, such as when a speaker says:

I'm not [the] least surprised that she's leaving.

if you leave in the "the" it sounds more general; it's pretty much a fancy way of saying "I'm not surprised she's leaving." But if you drop the "the" it emphasizes the comparison, and I would start looking around the room for someone less surprised than the speaker (or just realize they are a non-native speaker).

As far as practical usage, "the" seems like the safer course as John Lawler says, unless you are emphasizing a comparison. As a speaker of US English, I would tend to add the "the" in all of your examples except "I'm not [the] least surprised that she's leaving" (for which I would actually say "I'm not the least bit surprised that she's leaving", or even more likely "I'm not at all surprised that she's leaving"). Leaving the "the" off for all the others sounds slightly more stiff and formal.