(1)
Let’s examine this first sentence that you presented for analysis:
“Better too much courtesy than too little, provided it is not indiscriminate, which degenerates into injustice.”
It’s a grammatically and structurally faulty sentence that has an inverted main clause that’s also elliptical, followed by a confusing sprawl that consists of a participial modifying phrase (“provided it is not indiscriminate”) and a dangling relative modifying clause (“which degenerates into injustice”).
The normal, full-bodied form of that main clause is actually the following:
“Too much courtesy is better than too little of it.”
Its full, straightforward form is, “Too much courtesy is better than too little [of it],” with the words “of it” dropped or ellipted. And to make the statement more emphatic, it was also subjected to a sentence restructuring called inversion, which brought the adjective to the front of the sentence, as follows: “Better too much courtesy than too little.”
Then that whole clause was modified by the participial phrase “provided it is not indiscriminate,” thus taking this form: “Better too much courtesy than too little, provided it is not indiscriminate.” The comma that sets off the participial phrase from the main clause, evidently used for emphasis, is actually optional. In any case, all’s well and grammatically airtight with that entire construction by far.
But that construction runs into serious trouble when the modifying relative clause “which degenerates into injustice” is added to its tail end, as follows: “Better too much courtesy than too little, provided it is not indiscriminate, which degenerates into injustice.” This is because that modifying relative clause can’t logically link or latch on to any subject in that sentence. It’s obviously not modifying the adjective “indiscriminate,” and it’s farfetched and grammatically incorrect to assume that it’s modifying the main clause “better too much courtesy than too little.” In short, that modifying relative clause is a dangling modifier.
This is a rather complicated state of affairs that normally can’t be corrected by conventional grammatical fixes, but there’s a perfect but grammatically advanced solution to it: a summative modifier. See how that grammatical fix works:
“Better too much courtesy than too little provided it is not indiscriminate, in which case it degenerates into injustice.”
Here, I used “case” as a summative modifier to refer to and sum up the participial phrase “provided it is not indiscriminate,” then used “it” as a pronoun to refer to the noun “courtesy” in the main clause. This grammatical strategy neatly eliminates the dangle and logically connects the prepositional phrase “in which case it degenerates into injustice.”
Now take a closer look at the noun “case,” which works as the summative modifier in that sentence. As you can see, a summative modifier is an altogether new word or phrase that’s used to sum up a core idea of the preceding clause, then makes that word or phrase the thematic subject of succeeding relative clauses. The summative modifier positions itself right after a pause created by a comma at the end of a sentence segment. In this particular instance, “case” recapitulates the idea of the participial phrase “provided it is not indiscriminate,” and, in turn, the relative clause “in which case it degenerates into injustice” elaborates on it with new information.
(2)
Now let’s analyze the second sentence in question:
“It costs little and helps much—everyone is honored who gives honor. Politeness and honor have this advantage, that they remain with him who displays them to others.”
Yes, the clause “everyone is honored who gives honor” is the grammatical equivalent of “everyone who gives honor is honored,” which is the normal construction. The alternative form used in that sentence, “everyone is honored who gives honor,” is called a discontinuous noun phrase construction. This is because the noun phrase “everyone who gives honor” is broken into “everyone__________ who gives honor” to give way to an earlier appearance of the verb and predicate, “is honored,” such that it reads as follows: “everyone is honored who gives honor.” By being delayed until the end of the sentence, the modifying phrase “who gives honor” gets greater emphasis or stronger focus.
Now, regarding your question as to what the word “that” is doing in the following sentence:
“Politeness and honor have this advantage, that they remain with him who displays them to others.”
The word “that” in the sentence is a conjunction that introduces the subordinate clause “they remain with him who displays them to others,” which, of course, describes or modifies the noun “advantage” in the main clause.
(3)
You ask why the sentence below doesn’t have a subject:
“Therefore pay respect that you may be respected, and know that to be esteemed you must show esteem.”
That sentence is in the imperative form, which, I’m sure you know, normally omits the subject and doer of the action and sometimes even the receiver of the action, as in the command “Attention!”, which is the succinct form of this complete sentence: “I am ordering you to pay attention!”
In the same token, the sentence you presented above is the abbreviated, emphatic form of the following completely spelled-out sentence:
“Therefore you must pay respect that you may be respected, and you must know that to be esteemed you must show esteem yourself.”
