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."

For example:

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

Ha is when you want to specify the topic. No is used to discribe the possessiveness of something. Try the youtube. Lot of interesting material there.

Since the subject of the sentence is the older brother, the 'ha' comes after him.

I think they expect you to know the Japanese schooling system. Grades 1-6 are [小学], Grades 7-9 are [中学], Grades 10-12 are [高校] and Grade 13 onwards are all considered [大学]

You would use the kanji for 5 (五)...

It's a polite prefix to add to many nouns (お皿、お兄さん, etc) which is actually a form of keigo.

Same. @.@

It's also important to remember that in the vast majority of cases, Japanese doesn't use pronouns. Since かのじょ also means "girlfriend" (and かれ = "boyfriend") you may hear the word more in that context. Duolingo seems to just be using it for the sake of convenience.

It's for both younger and older females. (As for me, I usually hear older males refer to younger girls as kanojo.)

seems like you forgot the は after おにさん

It lacks the "は" particle but I suppose you solved that much time ago

you forgot about 「は」between 「おにいさん」and「小学五年生」

you forgot about 「は」between 「おにいさん」and 「小学五年生」 ：）

小学 You have to type it out in hiragana しょうがく it will then turn into 小学. Try it again. Make sure ょ is small not よ.

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.