いえ is definitely used informally for いいえ, though it's not super casual. More casual options include いや, ううん, or 違う. That being said, you don't necessarily need a word that means 'no'; you can reply to 「フランスに行ったことある？」with just 「ないよ」
Since there is no real universal rule regarding how to distinguish two big brothers or sisters in form of address, each family might have their own way, but one common pattern is [nickname] + 兄【にい】/姉【ねえ】. The [nickname] is often their given name shortened into two morae long. The suffix also could be 兄さん, 姉ちゃん etc.
Calling senior siblings お姉さん or お兄さん ...
Of course a sentence like 鳥は飛ぶ ("Birds fly.") is 100% grammatically correct. It's 100% standard, too; no slang/dialect is concerned. I'm not an expert, but you never need a professor nor a linguist to judge the correctness of such a simple sentence. Don't be that skeptical about your textbook.
But did your textbook really call this casual style? I ...
The simple answer is that if you want to learn natural spoken Japanese, you need to focus more on authentic source materials like movies, podcasts, TV series, social media posts, or native-speaker-generated example sentences. These days, online translators use neural machine translation - it's definitely getting better and might eventually be able to ...
The informal version of "彼女はいつ来ますか？" is "彼女はいつ来るの？"
"のか？" is a literary style, and it almost never happens in ordinary conversations in the real world.
BE + -ING (eg "She is coming tomorrow") is an English way of expressing future tense. Japanese ている doesn't have such a function.
1.Yes, it's a gender-neutral expression.
When used by a man, you can change the first person to mean the same thing, as in "俺なんか" or "僕なんか".
2.Depending on the sentence that follows it, it can sound pessimistic.
When you use "私なんかダメな人間だ" you are saying that you can't see yourself as a good person.
Well, there are many types of 略字.
Short answer: it might be used on blackboard.
Generally speaking, I don't think primary school teachers use them, and also unlikely secondary school teachers. I remember my history teacher in high school using 口 for 国. But it was rather a memorable exception. As for essays, ...
I'd say 彼女はいつ来るの? Just adding の right after the base form of verb works. This rule can be applied to other verbs like 戻る, 帰る, and so on.
However, Japanese don't use 彼女, 彼 or 私たち so often. I actually pictured myself speaking and figured out that it would be more natural if we omit subjectives or specify persons' names. e.g. いつ来るの?, Mary はいつ来るの?
だから = だ (casual copula) + から (because)
だから is basically the casual form of ですから. You can't use です with a verb in ます form, so ですから wouldn't work in the formal version and だから wouldn't work in the casual version since だ is the copula and you already have a verb - 食べる.
The word itself has been widely recognized for a long time as "something you shout in the mountains". Even novels written in the 1950's have examples of ヤッホー.
In town, it may be used very occasionally as a humorous, unique greeting. Well, sometimes people feel おはよう is too uninteresting and want to say something different. I may have heard ヤッホー used ...
No one says yahoo in Japan.
Some decades (around 100-70) ago , Yahoo was used for making echo or voice call someone in the distance at mountain site. Yahoo is recognized as a calling in laud voice for distance , so It may use for joking. Never used as a popular greeting.
Decent people does not yahoo for greeting.