"I am John."
It would be helpful to hear the options as you press them as with the other languages in this app
It's implied the speaker is referring to themselves. A lot of Japanese is implied. If you were introducing yourself, "Hello I am John," then "Watashi wa Jon desu" would be appropriate. If someone asked you your name, you both already know you're talking about yourself, and you can leave Watashi off.
Watashi is gender neutral, it can be used for either males or females. Boku is a more casual way of saying "I" when you re a male. A female would never use boku, but that doesn't mean that Watashi is only for females.
Boku is also, as I mentioned, a more casual way to say "I" as a male. You shouldn't say something like "Boku wa Jon desu" to someone you've just met, or use boku around a superior, it would likely come off as arrogant or rude.
Really it's just a safe bet to always use watashi. It doesn't sound overly polite, and most of the time in Japanese you can omit saying watashi or boku anyways, so it's pretty easy to not worry about it.
Desu is a very strange concept for English speakers, I feel. I honestly think it's best when first learning Japanese to not worry about what it really means, and just use it when you feel it is correct.
To put it simply, desu is a generally polite way to end a sentence. I could introduce myself to you and say "Watashi wa Kyle," and that's completely fine, but it may sound a bit... short?
Imagine it like this: In English, if we were introducing ourselves to one another, I would probably say something like "Hi, I'm Kyle, nice to meet you" or "Hi, my name is Kyle." If it was a fairly casual or informal situation, I might just say "Kyle" and shake your hand, or something to that effect. Leaving off desu is kind of like that (in this instance), it just makes things more informal and short.
So what does desu mean? Again, don't worry about that at this point if you're new to the language, it doesn't really have a great English meaning. I tell people to almost think of it like the period on the end of a sentence. If you're not ending your sentence in a verb, for now go ahead and end it in desu instead, just as a nice way to wrap up and conclude the sentence.
Would this be literally translated to "Jon" and the "h" inferred since "John" is the common name, or is there a grammatical rule that makes that "h" legitimately appear?
Japanese uses Katakana as their alphabet for foreign words, or for sounds that don't exist in Japanese. The "h" is not inferred, as no single "h" exists in Katakana. Names are often changed to fit into Katakana. For example, if your name was Tony, you would most likely be called "Toni" in Japanese. "Ny" doesn't exist as a Katakana, so "ni" is used instead. In names where no equivalent sound exists, you attempt to replicate the sound with what Katakana are available. For example, my name is Kyle. "Ky" does not exist, so it is changed to "Kai," the characters カ (ka) and イ (i) are used to replicate the sound. The second half of my name, phonetically, is "uhl." No such sound exists in Japanese, and honestly nothing even really comes close. Instead, ル (ru) is often used to substitute the sound, making my name in Japanese "カイル," or "Kairu" if you spell it in Romanji.
Names do not have hard rules, and people with foreign names can often get away with calling themselves what they want. Some more common names have much more "set in stone" translations, however. Finally, if you convert your name into Katakana and it ends up being (phonetically) the same as a Japanese word, most people will change your name. Again, not a hard rule, but imagine someone visiting our country who was named "Milk," or "Briefcase." Generally, you would probably call them by a nickname in that situation, or that person (if they're aware of this) will often opt for a different pronunciation of their name.
It seems odd to me that "じよんです" doesn't count as correct for this prompt, but "まりあです" worked perfectly for "I am Maria" in another prompt in the same exercise. How is じよん different from ジョン besides being of a different alphabet / set of characters / whatever it is?
phonetically.. you have written "jiyon" in your post when what may have been accepted in hiragana would have been "jon/jyon"
( the よ should be small )
Your comment reads something like "I am before am John." I think what you were trying to say is "Watashi no namae wa John desu," which reads "My name is John."
Watashi wa John desu = I am John
Watashi no namae wa John desu = My name is John
John desu = John
"Watashi no" is a great phrase to learn, and effectively means "My." For example, "watashi no keitai" just means "my cellphone."
A final note, as a general rule, you shouldn't have more than one "wa" in your sentence. There are of course exceptions to this rule, but at this level it is very unlikely this will occur.