Hi, actually these are the complete sentences 
One of them is the title of the page I believe.
I would have never thought of it being な + んだ + し
Firstly I did not expect 15 to end with な but it makes sense of course and i just did not learn the し yet.
There is a ている past form? I guess I will learn it in N4 or later? It seems weird to me that it is not included in N5.
Thanks a lot for the explanation. It helps a ton.
I know I said “at least the full sentence”, but the point is to provide enough of what’s being said in order to get the full context. There are times when that’s going to be useful but, again, not in this case. Don’t get hung up on semantics 
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Anyway, I’m glad that was helpful.
As for the ている form, there are 3 explanations of it in BunPro. They’re all N5 level. If you look at the yellow warning triangle right at the bottom of the title of the grammar point, you’ll see that it says:
“ている is often shortened to てる. This applies to all tenses, including てる, てて, てた, and even てます.”
All 3 have them. It’s been there the whole time, you simply missed it.
yes you’re right. I totally missed that. I still would have expected an extra grammar point for it because the meaning has to be different to. It has to be something that was in a state but not anymore or something instead of in a state and still in. There are several points splitt up for positve and negative or non past and past so I think it is not crazy to expect that.
But now I’m aware and I will keep my eyes more open to the yellow parts 
Thanks again
Don’t ever. Use DeepL instead if you need to, or ask Bing chat in precise mode.
This N5 lesson explains noun+な+の
Things are kinda over-explained at N5 level since everything is new but as you go on you need to start to work under the assumption that whatever you read in a grammar resource is just a guide and not the whole picture (especially beyond N3 level).
When posting a Japanese sentence or word it is best to provide as much context as possible due to the fact that Japanese drops subjects/objects/as much information as possible. To give an example in English (which is a far less contextual language): Since it was heavier than the other he didn’t wanna give it to his son. I can easily imagine a learner of English asking “Since what? What is ‘it’? Who is ‘he’? What is the ‘other’? Why is ‘the other’ and not ‘an other’? How can you say ‘he’ and ‘his’ in the same sentence? Isn’t that confusing?” etc etc. Problems like this are even worse with Japanese. The sentences you posted clearly have a context as they are explaining reasons/excuses for something and the second one has to have some subject but it is impossible to know who or what the subject is without the context. If you are at the stage where you can’t parse a basic sentence then it is best just to post as much information as possible as the solution to the problem may lie outside of the sentence itself.
I personally use both. Sometimes DeepL doesn’t translate correctly, and vice versa. It depends on the sentence structure.
I read the first as something like,
Well, because I actually already said it 
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And the second as something like:
I wanted to eat soba.
Actually, the first sentence says:
“Well, It’s because Yuno is also already 15 (as one of the reasons for something).”
The second one is good 
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so し at the end of a phrase means “because”?
If that し is acting as a particle, it can mean “because”. In that case, it frequently lists multiple reasons for something. For more information, do check し～し (JLPT N4) | Bunpro – Japanese Grammar Explained
Where is the “wanted” coming from? I read it as “because I was eating soba”, but I’m a noob.
if it is the ている past form I would expect that as well.
I completely missed that. You’re right.
I don’t know why I completely missed read it the second time, after I had just explained that it was the past tense of the ている form. You’re right.
Lol I did not know that was a character, I thought it was just the thing where in very casual speech speakers say ゆう instead of いう」
Also as far as 食べてたし I could be wrong but it felt like 食べていたし at first, but I was going off of the Jisho def where it says ‘want to …’ etc. that is the たし grammar, I could be wrong. The simple ‘I was eating’ seems equally right.
It just seems like if you were going to say I want to eat you would just say 食べたいし、so past tense felt right. I could be totally wrong. This very contracted casual speaking is not a strength of mine. Again the greater context would help to clarify that.
edit:
Looking it up more it seems like 食べたい→食べたし which was where my confusion was coming from. not sure if that applies to the past tense or not. I’ve seen version of this い形容詞 coming from しく形容詞 but I have not really mastered it. I will ask my Japanese Teacher Friend next week.
I just assumed that it was a character in the manga, given that the sentence didn’t make sense to me otherwise.
As for the たし part, to be honest, I had never heard of it until someone mentioned it earlier in this thread. It seems to be a really old version of the たい form, but the たい form doesn’t have a “Progressive/Continuos” conjugation (for lack of a better term). At least AFAIK, these are the ones you typically see:
食べたい = I want to eat.
食べたくない = I don’t want to eat.
食べたかった = I wanted to eat.
食べたくなかった = I didn’t want to eat.
I could be wrong, but I’m assuming it is the same way with the old たし conjugations. If so, then the only thing that makes sense here is that the “Progressive/Continuous” conjugation, plus the し suffix, is being used in the informal past tense. As in: 食べて(い)たし. That’s my thought process at least.
The し～し grammar point could do with a few examples where it comes at the end of a sentence, since that’s generally how I see it in manga. Either that or give it its own grammar point since its usage gets closer to から/ので when at the end of a sentence than when not.
Instead of creating yet another grammar point for it, a better solution would be to add that し can be found at the end of a sentence somewhere in the information box.
That said, I feel like even that is unnecessary. Don’t think of Japanese as this rigid language that can ONLY work in a certain way. Admittedly I’m sometimes guilty of this myself, but I adjust after realizing how a grammar point or word can be used in other ways. It’s mostly common sense .
-し is the classical/literary 終止形 of adjectives that doesn’t exist as a separate form anymore in modern Japanese, but it’s still commonly used with some words, especially 無し.
-たい can be used in this way. ソバを食べていたい means something like “I want to keep eating soba”. I don’t think it’s a likely thing for someone to say but it is possible.
But classical conjugations and leaving out the い in ている don’t go well together. Another thing is that 食ぶ (where 食べる comes from) is actually a humble expression for 食ふ (modern 食う). So when you see 食 together with -たし e.g. in a saying, it’ll likely be 食いたし and not 食べたし, for example in 河豚は食いたし命は惜しし.
Thanks for the detailed explanation 
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wow, I don’t get half of that forms yet, but it is still very interesting.
By the way I was looking if there is an official translation but it does not seem so.
But
) in the 4-koma before and referring to it in the current 4-koma. So “But we were eating soba” or something like that makes totally sense.