"Her older brother is a fifth grader."
someone can correct me if I'm wrong, but my Japanese teacher (born and raised in Japan) said they say what year of what level schooling they're in. so, for example, a freshman in high school would be 高校一年生です。and a sophomore would be 高校二年生です。 and so on. "middle school" starts at 6th grade but it would be 中学一年生 up to 中学三年生。
I'm glad you asked this, because this is one of the features of Japanese that makes the language beautiful.
So in attempting to read a string of complicated characters, some of the questions one would like to answer is "What is this sentence talking about? What is the subject of this sentence? What is the subject doing? What is the subject doing something to?" and "What are the relationships between subject/noun and objects in the sentence?"
In Japanese some Hiragana symbols are used syntactically in ways which help clarify these questions, and are referred to as "particles." So far up to this point in Duolingo you've seen quite a few of them. Namely:
ね - used when looking for agreement or confirming a statement
Example: 寒いです！ "It is cold!" そおうですね "Yes it is."
は and が are used to mark the "topic" and "subject" of the sentence respectively. It is important to note that the "subject" of a sentence has a grammatical relationship only to the verb, while the topic is a non grammatical context for the whole sentence.
The difference between these two is highly nontrivial, and a good explanation is given here: https://japanese.stackexchange.com/questions/22/whats-the-difference-between-wa-は-and-ga-が?newreg=3675ce533daa412aa5ae1676a45f834d
But for now, a good heuristic is to think of は as marking something already familiar to both speakers in the conversation.
For example, if a friend visits your family and sees your brother, both speakers would know who "he" is in the following sentence
彼は私のおにさんです "He is my older brother."
Now suppose while in the conversation, you observed a cat outside behind the person you are talking to eating a bird. In this situation if you mention a cat with は, this is assuming the speaker already knows which cat you are referring to and would be incorrect. Here we would say:
猫が鳥を食べます "The cat eats the bird"
More detail about these including other uses for both か and が can be found at this link: https://thoughtco.com/japanese-particles-wa-vs-ga-4091105
の - In the beginning contexts here is used to indicate possesion. When you see "AのB," just think of the grammatically correct way to say "A's B."
彼女の猫 "Her cat"
ブランドノの犬 "Brandon's dog"
の also has various other uses which are explained here: https://thoughtco.com/particles-o-and-no-2027923
Well that is mostly true I feel like for most of the new learners telling them that is wrong because they never figure out the specifics. It's like saying there's no rules so then you can't actually explain it when you need to.... I would say that this language is flexible but not without parameters. I honestly I don't know how people get this far in the lessons without a basic understanding of wa and ga
The problem is not about knowledge of Japan. The problem is that Americans say that someone is a "fifth grader" without explicitly answering the question, "Fifth grade of what?" We are just expected to know that Duolingo means "in the fifth year of primary school" here, without being told. If it had said "in the fifth year of primary school" in the English sentence, it would have been immediately clear that it would be 小学, and not 中学. So the problem is not a lack of knowledge of the Japanese schooling system but a lack of clarity in the English sentence.
You gotta visualize the romaji for this one.
Rather than saying "shogaku", it's written (and pronounced) as "shougaku".
So the kana looks like しょうがく and if you hit space or enter afterwards, it should come out as 小学。
As a side note, I had to hit spacebar twice to shuffle to the correct kanji.