“could not distinguish”
Sometimes I’d treat “could distinguish” as one node for simplicity.
Some people might treat “could not distinguish” as one node but I don’t like that. I think the clear, non-verb modifier “not” should be separated.
If you split them, some grammarians would put “could” as an auxiliary verb child of “distinguish”. I object to that on fairly technical grounds, although I grant it’s reasonably understandable.
I would split them with the finite verb (could) as the parent. “Distinguish” is a bare infinitive serving as the noun object of “could” so I wouldn’t label it a “verb”. I’d give it a label which distinguishes it from a finite verb. That could be “noun” or for more clarity e.g. “verbal noun”, “infinitive noun” or two colors (in some videos I use a colored dot after a word for a second color). I think “distinguish” is more noun-like than verb-like here.
With the nodes split, I think “not” should apply to the main, finite verb, “could”. I think it’s saying he was unable to do something – the opposite of being able to do it (can = be able to). He couldn’t distinguish (unable to do a positive action) rather than could not-distinguish (positively able to do a negative action).
“ebbing”
“Ebbing” is another non-finite verb. It looks a lot like a noun/gerund like Max says. You can think of sentences like “He understood the cause of the ebbing.” where it’s clearly a noun.
But you can also think of sentences where it’s a modifier like “The ebbing water was cold.” I think that’s like “toy soldier”, “metal toy”, “history teacher”, “race horse”, “boat race”, etc. See e.g. Noun as Adjective | Grammar | EnglishClub and Nouns as adjectives | Britannica Dictionary which names them “attributive nouns”.
But is “ebbing” an attributive noun? It fails some of the tests Britannica’s editor gives for identifying attributive nouns:
(a) they cannot be used predicatively (“the time was turnaround” is impossible),
“The tide was ebbing.” is fine. Maybe that’s because it can be a regular noun in addition to being an attributive noun, and in this sentence it’s acting as a regular noun?
But note that “ebbing” isn’t plural yet no article is needed. Compare to “The object was a toy.” where “toy” requires an article when acting as a regular noun. (Remember from “toy soldier” that “toy” can be an attributive noun or at least serve as some sort of modifier.)
I think the article issue may be irrelevant. It’s a conceptual gerund, like “running”, which refers to the concept of running. Concepts like “chemistry” don’t need articles either. “The subject was chemistry” or “Chemistry was fun” both work (they also work with running instead of chemistry). I think the issue here is just that it’s not a count noun.
(b) they cannot be premodified by very (“a very turnaround time” is impossible),
Test passed.
(c) they do not take comparative or superlative forms (“turnarounder time” and “the most turnaround time” are impossible),
I think “the more ebbing tide” would be OK if you were comparing two tides. Or “my most ebbing emotion is jealousy” seems OK. They’re awkward though.
(d) they cannot be made gradable by modifying intensifiers (“so turnaround time” and “highly turnaround time” are impossible).
“highly ebbing tide” seems OK, though awkward.
So one test for being an attributive noun clearly passes, one clearly fails, and the other two might fail but only with awkward examples. I don’t know how good those tests actually are. You definitely can’t say “most running” or “highly running” so maybe “ebbing” is different somehow. A synonym for “ebbing” is “receding” and you can say “highly receding” or “most receding” (I think; again, seems awkward); I think it’s just that some actions come in degrees and some don’t.
I’m not sure why “highly receding” seems merely awkward to me while “very receding” seems dead wrong. “highly” and “very” have nearly identical definitions.
Knowing the term “attributive noun”, it’s easy to look up more info like Attributive Noun Definition and Examples and Help | Merriam-Webster. “FAX TRANSMISSION NETWORK ACCESS COST OPTIMIZATION PROPOSAL” is a neat example. And one of those pages gave more terms that could be searched: “noun premodifier”, “noun adjunct”, and “converted adjective”, but I’ll stop here.