Hello everyone!
I have just finally logged back in to Bunpro to reset everything and start from scratch.
I believe there was a grammar point for ga-no substitution (when you replace が with の in subordinate clauses). I looked for の in the search but I just can’t find it. Was it ever there? Hope someone can help me find it 
I’m not really sure that needs its own grammar point? It’s just a change in how the sentence is phrased. 私が食べたケーキ “the cake I ate” vs 私の食べたケーキ “my cake (which I ate)”
N5 Lesson 1: 8/12
There has never been a dedicated grammar point for this, although it is mentioned in the writeup for the possessive の. However…
We actually already have a draft for this specific grammar point ready behind the scenes and should be publishing it soon!
At an N5 level the usage is probably not very intuitive, and understanding how this particle swapping works also helps with understanding how relative clauses work (something which can be a big key to understanding more complex sentences as a beginner). There are also a couple of grammatical nuances worth explaining beyond the basic substitution, so we felt that this is deserving of its own grammar point for an N5 level learner.
yay!
this is also something that I’ve wondered about from time to time, so I’m excited to devour the new grammar point 
Wow, that is great news! Looking forward to it being published! 
It looks like it’s already available via the Grammar Library. I learned it yesterday and found it to be very helpful and informative!
Its (usually) not a substitution at all.
I mean I guess its a substitution the same way replacing any particle with any other particle is “substituting” it.
But at the end of the day, its just a completely different particle, doing its job as a particle, with the nuance of that particle.
First I thought this post was talking about the actual case of が being substituted by の. As we know, in older grammar, が literally substituted for の; “我が家” = “我の家”.
One difference in the sentences lies in the predicates which are being referred to to embed the larger clause into one main and one relative-clause:
A (⓪が)(私が食べた)ケーキ(である)
B (⓪が)私の(食べたケーキ)(である)
In B, の refers to the ケーキ (or rather the omitted copula). We are saying “It” (⓪が) is (である/だ) my (私の) eaten-cake (食べたケーキ); Here, 食べたケーキ is the relative (aka embedded) clause. Note that the Subject (who has eaten the cake) is implied; “I” am not the Subject but may be implied to be. Whether the cake was eaten by me or by someone else remains ambigious. Thats very typical for Japanese.
Although note that based on pronunciation, 私の(食べたケーキ) can also function as (私の食べた)ケーキ in which case this is closer to a substitution of が. Though the difference is subtle, we are still not declaring “I” as the Subject. If I had to translate, it would be like saying “The cake eaten by me (Lit. “The cake of my eating”)”, and not “The cake I ate”.
In A, 私が refers to the verb 食べた and builds a relative clause with it;
“It” (⓪が) is (である/だ) a I(Subject)-have-eaten (私が食べた) cake (ケーキ). Here, the Subject is clear but of course we do not mention possession–so whether it is “my” cake or not remains unknown.
I have never thought about it this way, so thank you for sharing!
I guess this is what @prolezone ment by
Also, you provided this very interesting example:
I didn’t know you can use の with verb forms this way!
There is this example in が・の (Subject):
天気のいい日が好きだ。
I like days when the weather is good.
If I try to apply the reasoning you suggested to this sentence I get something like
天気の(いい日)が好きだ。
I like weather’s good-days. / I like good-days of weather.
as opposed to
(天気がいい)日が好きだ。
I like weather-is-good days.
Do you think these accurately convey the difference in the nuance?
Seen in this way, they sound quite different to me because in the first example いい describes 日 but in the second example いい refers to 天気.
Leaves me wondering if they are really interchangeable!