"I eat at around twelve o'clock."
Because へ is used for movement, for example 日本へ行きます(I go to japan).
Movement is only one of the many functions of に, so I could also say 日本に行きます, or for example, 部屋に猫がいます(There is a cat in the room), which I use に to determine the place where the cat exists, that へ Can't be used for because it is only used for movement.
In this phrase, it was used to determine the moment, period in time you eat.
As well as such, keep in mind when they say "pretty much interchangeable" they mean in the sense that "ni" is to square as "he" is to rectangle. Every rectangle is a square, but not every square is a rectangle. So, really "ni" is the only one really interchangeable. But, "he" can be used in place of "ni" at times. Like Amodeus_R. said, "ni" is used for location/time/movement of something that is or will be. While, "he" is used mainly just for movement.
Yes, I agree, when the intro says "Don't worry about it, they're pretty much interchangeable"... It's like they don't actually know what interchangeable means. They should have said that you can always use ni and in some cases you can use he. Their explanation of the difference between the two was super unclear too.
ごろ is typically typed out using kana only, not kanji. Kanji exist for a lot of words, but just because they exist doesn't mean they're common in everyday use.
Also, just a note, you mean kana, not furigana; furigana is when you see kanji with small hiragana beside it as a reading aid.
You don't need to change anything if it's obvious from context that you're talking about the other person. For example, if you were having a conversation with someone about lunch and you phrased it is a question (just adding a か at the end), it would be obvious you meant them and not yourself, so the same sentence works.
To be completely explicit about someone else, you'd add あなたは or (their name)-さんは (or whatever honorific is appropriate) to the front of the sentence. Using their name is more polite and you usually wouldn't use あなた.
Well, the second option is a more correct way of writing 12 o'clock in Japanese. The first one contains a katakana character ニ pronounced 'ni', the same way as the number 2 in Japanese. It may depend on the font, but katakana characters are generally smaller than kanji.
If you wrote it by hand, it wouldn't make much of a difference, cause those characters look almost identical and everyone would assume it's the kanji for 2, not katakana. But here they are coded differently and cannot be used interchangeably (if you search them in an online dictionary, you would get two different results). Using katakana like that doesn't make much sense in an everyday writing. If you don't know a certain kanji, it's better to use hiragana in it's place (so, in this case, 十に時; though じゅうに時 or じゅうにじ look more natural).
を is what the object is or to what the action was done. In this you need に which in this case will express clock times, it can also express days of the week, months, or years.
(It is also used to express the destination of an action involving movement or the location where something is.)
So this is not for this specific lesson but for Duo in general. Days ago I finished the last level one for the language and got a golden Duo as a reward but I still have not recieved the achievement page award. Does anyone know how to contact Duo to see what the problem is?