Yes, there should be a comma (in my opinion). If you could use an 'and' (a conjunction) to join the words but choose not to, normally you need a comma.
In your example, after 'so deep' you could choose to write 'and so disappointing' so you should have a comma.
A general grammar rule is that when joining two independent clauses with a conjunction you ...
From my point of view, the way we write should convey as close as possible what we speak. So even if there are no clear rules sometimes, or even in spite of some rules that hinder the use of language, I have some small rules (of my own) to decide how to write.
In the context of your question, I use the rule: does the sentence have (more or less) the same ...
along the way is an idiomatic phrase. See below, for others.
I see no reason to get tied up in the words parenthetical or non-essential as this question is a matter of style, and, not a grammar question.
The phrase is often used.
But I would call it an adverbial phrase since it answers the question:
When did their love start to fade**?
Then, their love ...
Questions about punctuation depend on what style guide you adopt. There is no universal agreement on punctuating English.
My personal guide is whether there would be a pause if the sentence were spoken. I would say
Then brief pause along the way brief pause their love started to fade
so I would write
Then, along the way, their love started to fade.
I would say that "along the way" is parenthetical, or non-essential.
Then their love started to fade.
would be a valid sentence, with the same basic meaning, although without an added nuance or detail.
The other example in the question:
Then, without even noticing it, their love started to fade.
uses the same structure, although the meaning is not ...
Using a full-stop here is a small artistic liberty. Even though the sentence is grammatically formed as a question, the full-stop makes the dialogue read like a statement. The speaker is asserting that what he said is true, instead of interrogating the listener and expecting a reply. It is a rhetorical question. In spoken language this distinction can be ...
After having a green salad and black coffee for breakfast(,) and putting on her white cap and black hoodie, Mary went to university to meet her professor.
That sentence can be improved by adding then.
After having a green salad and black coffee for breakfast and then putting on her white cap and black hoodie, Mary went to university to meet her professor.
The two introductory clauses are independent - because she is having breakfast and putting on her hat and coat.
You wouldn't need a comma if you were saying:
After having a green salad and black coffee for breakfast and a bagel and orange juice for lunch, she was quite full.
This is because the two clauses are both about eating.
I think the comma ...
Disclaimer: I'm not very sure about this, so correct me if I"m wrong.
I did a research and found this website. In it, they say:
The adverbs of concession set up contrast clauses [...] Use commas to
introduce dependent clauses beginning with these words, even when the
independent clause comes first.
Or at least isn't in the list of adverbs of ...
Options are generally separated by commas.
You can have chicken, beef, or fish.
In your example there are two options: either you have read the book, or you have just heard of it.
You have probably read The Girl with the Dragon Tattoo, or at least heard of it.
The two options are correctly separated by a comma.
With only 2 items the rule is clear: without the comma.
With 3 or more items the situation about the comma before the final “or” (or “and”) is more complicated:
British English allows constructions with or without the comma.
In American English the comma is almost mandatory.
In Canadian English the comma is almost forbidden.
Australian English allows ...
I think this is more a matter of style than grammar.
Sometimes a comma is required, and other times it's optional. Here, the comma indicates a pause in the sentence. One might wonder if the pause is mandated or not. The example sentences strongly tend towards their current form. "It was so big it was scary" (without the comma) is like someone speaking ...