Chapter 16
Page 87, Panel 2
This usage of いや is the following definition seen in this Japanese dictionary entry:
Essentially, you say いや when you are taking back something you just said.
In this case, Kotoha said the reason Sacchan was in trouble, but then she takes it back and suggests it was this other thing Sacchan did.
In English, when we want to take back what we said and replace it with something else, we might say, “no, wait” in-between.
For example, “Sacchan got in trouble for doing such-and-such. No, wait, it’s because she did this other thing.”
Page 87, Panel 6
〜ことにする is commonly used grammar which has a meaning similar to “to decide to” in English. You’re doing (する) toward (に) something (〜こと). Essentially, “I will do (toward) 〜”
Next, we have the verb 置く, meaning “to put”. When you take a verb + て + おく, it means you’re doing an action and leaving it there.
In some cases, you’re leaving the action there to go back to later. Such as, make a reservation and leave it at that, and you’ll make use of it when you get to the restaurant tonight. Or memorize a speech and leave it at that, and you’ll give the speech at an event this weekend.
This て + おく will often become とく in speech, which we see here.
する + て + おく ➨ して + おく ➨ しておく ➨ しとく
Kotoha is essentially saying, “Well, we’ll leave it at that.”
In this case, it’s not necessarily that they’ll come back to it later.
Context: Yui wishes she went to the same school as Sacchan and Kotoha. Kotoha asks if Yui doesn’t have any friends (at her school), to which Yui says she does. Kotoha responds with a, “Hm…” which suggests she’s questioning whether Yui’s telling the truth.
When Yui doubles down, Kotoha suggests they leave it at that, seemingly still in disbelief that Yui actually has friends at school.
Yui gets mad because Kotoha doesn’t believe her (or perhaps that Kotoha suggested that Yui can’t make friends at school), so Yui borrows the game to get back at Kotoha.
Page 88, Panel 5
かける is one of those words that’s extremely versatile. As you’re probably seen looking in a Japanese-to-English dictionary, it gets over 25 different English counterparts.
And if you check a Japanese dictionary, it’s even worse!
Looking there, I see five definitions that can use the spelling 欠ける. Then another 32 that can be written as either 掛ける or 懸ける (and many of those have multiple related uses). Then 3 uses for 駆ける, plus another and 3 for 駆ける. And finally, 3 uses for 賭ける.
Overall it’s best to get to know かける over time (as you see it used in many ways), but Cure Dolly has a nice subtitled video discussing かける.
One of these many uses is to state the amount of time spent on something. For example, saying the moon takes (approximately) 28 days to go around the earth, this would be 「28日かける」.
Or, in Kotoha’s case, she’s spent three days (3日かけて) playing her new game.
やてんだ comes from:
やる + て + いる + の + だ ➨ やって + いる + の + だ ➨ やっているのだ ➨ やってるのだ ➨ やってんのだ ➨ やってんだ
This seems like a lot going on (and it is!), but it’s not unlike how in English saying “wouldn’t you” comes out as “wudnchu”.
We have やる, another generic term for “to do”, in this case used for “to play (a video game)”.
Then it’s in the て form with いる which is like going to “to play” to “to be playing” in English.
Then there’s the explanatory の, which I’ll loosely translate here as “so”:
“So you’ve spent three days and you’re still on the first level.”
Looking at the Japanese definitions page, I think this usage falls under definition 12 in the section that applies to 掛ける and 懸ける.
欠ける has uses along the lines of something breaking off (splintering), as well as something lacking. (While Kotoha’s gaming skill is definitely lacking, that’s not the use here 
)
Page 89, Panel 1
It took me a while to grasp verbs ending in てて.
It’s 「[verb]ている + て」 meaning “to be doing [verb] and (then) (do) …”
For やってて, it’s “to be doing (something) and (then) (do)”.
Because we’re in the context of playing a video game, this やる refers to that, playing a game.
ゆっくり tells how the video game is being played (leisurely). And since it’s やっている, she’s not saying “I play (games) leisurely”, but rather “I’m playing (this game) leisurely.”
The final て shows that her thought doesn’t end there, but her speaking does. “I’m playing it leisurely (and that’s why I’m still on the first level).”
In English, a similar conversation could go:
“You’ve been playing for three days, and yet you’re still only on the first level?”
“W–well, I’ve been taking my time, so…”
In this example (not intended to be a translation of the manga’s dialogue), you can see how the second sentence is trailing off as the thought is left incomplete.
Page 89, Panel 2
That’s about right. Note that this is two separate sentences. It might not be immediately obvious, but you can tell because of how spaced out the two sentences are within the joined word balloons. (But some mangaka might present one whole sentence split out this way.)
The 「なぜそんなことを」 is in response to Yui saying Kotoha will be a grownup by the time she clears the level (with how “leisurely” she’s playing it).
Next, the 「友達いないって言ったからか？」 is the “Is it because I said you don’t have friends?”
Kotoha essentially teased Yui as not being able to make any friends, so Yui is being vindictive and striking back by means of Kotoha being a gamer who’s really bad at easy video games.
(Incidentally, this portion is probably my favorite scene in the whole series.)
Page 89, Panel 3
Correct. 越す ＋ て ➨ 越して.
One usage for 越す means to go through difficult obstacles and get beyond them.
Correct. One can also go with “I’ll get past it for you” for perhaps a slightly more literal translation, but “I’ll beat it for you” is proper in English.
Page 89, Panel 4
From Maggie-sensei on uses of なんて:
- When you look down, belittle someone or something or lack of confidence.
It’d say this is the use here. She’s not simply saying, “Level 1 is easy,” but “Something like level 1 is easy.” There’s this underlying sense of, “How can you have difficulty with something as easy as this?”
(I still struggle from time to time to grasp the nuance なんて is adding to a sentence.)
Page 90
We have a minor time skip between Kotoha telling Sacchan to stop taking pictures and do something, and Yui crying. Presumably, Sacchan convinced Yui to return the video game to Kotoha, and Yui’s anger subsided. Now Yui realizes Kotoha probably hates her for what she did and Yui feels bad for having done it.