Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

My spellchecker insists on replacing 'furthest' with 'farthest'. I was under the impression that farthest is strictly speaking in terms of distance, whereas furthest is more abstract. A poster on this forum gives the following example: The further [quantity] you walk, the farther [distance] you get. What gives?.
The NOAD I had on my Mac Mini reported the following note, which is the same note found on the Oxford Living Dictionaries about farther and farthest. Traditionally, farther and farthest were used in referring to physical distance: the falls were still two or three miles farther up the path. Further and furthest were restricted to figurative or abstract senses: we decided to consider the matter further. Although farther and farthest are still restricted to measurable distances, further and furthest are now common in both senses: put those plants the furthest from the window.

Most grammar checkers are capable of detecting the the misuse of 'your' and 'you're'; providing the necessary correction. I'm curious though, is there any sentence that can be constructed where replacing 'your' with 'you're' keeps the sentence grammatically correct, but changes the meaning? If this is not possible, what grammar rules are in place that prevents these edge cases from occurring?.
a. I love you and your bananas. b. I love you and you're bananas. This particular case depends on the your/you're coming after an independent clause followed by 'and,' since its feasibility depends on functioning either as a second direct object or as another independent clause. It also depends on the noun serving either as a thing that someone might be in possession of (the bananas we eat) or as an adjective or noun complement describing a person (bananas=crazy). Easiest if that noun is plural or uncountable. The same sentence could be constructed with nuts, garbage, and other nouns which I'll let you brainstorm because my stormer hurts. You can also (and thanks to @Barmar for drawing attention to this) use verbs, in which case they act as a gerund with the possessive pronoun and a present participle with the 'you're.' As in: a. I love you and your cooking. b. I love you and you're cooking. Or: a. I despise you and your smoking. b. I despise you and you're smoking.

Overheard on an elevator today, I didn't realize it was him. Corrected by the know-it-all, He. 'I didn't realize it was he.' The know-it-all then went on a rant about how everybody is a dolt for not knowing that. I swear I have never heard this before in my life. Is the know-it-all correct? If so, why is 'I didn't realize it was he,' grammatically correct? What rule makes 'him' incorrect here?.

The 'rule' is that the verb to be, in any of its forms, is a copula that takes a predicate rather than an object, hence 'It is I' and other phrases beloved of English teachers in junior schools. There are various theories about the reasons for this, but the truth is that it has become a shibboleth to distinguish those who had 'a proper education' from the hoi polloi who just learned English as they went along. [Another such is to point out that hoi is the definite article in Greek, so 'the hoi polloi' is a tautology; please don't comment to that effect unless you wish to be mocked.] Nobody pretends that this rule is used in everyday speech; Barham's 'Jackdaw of Rheims' has a distinguished audience of prelates and clerics observe a jackdaw His eye so dim, So wasted each limb, That, heedless of grammar, they all cried, 'THAT'S HIM!-- showing that, though you are supposed to know that the 'correct' phrase is 'that is he', you are not expected to use it in moments of stress.