From olimo's comments below:
"...In definition-like sentences like "X is Y", we use "- это"."
"It[это] is often used in definition-like sentences. The function is similar to the English "is", although it is not the exact translation of the word. The literal translation would be sort of "A boy - this is a child". Sounds weird in English but perfectly fine in Russian."
""X - это Y" is a definition-like sentence, so you don't use it to describe a specific boy. It is like "A mouse is an animal", "Physics is a science", etc.
The boy is a child = Этот мальчик - ребёнок. Dropping "этот" would sound unnatural here. You can also say "Мальчик ещё ребёнок", like "The boy is still a child"."
Hi, english speaker here, but I think I am starting to get this:
First of all, you need to understand the purpose of the dash (and I have noticed that in duolingo, they use a dash, not a hyphen, which as far as I understand, in proper English they are actually two different things; "-" hyphen, "—" dash, but in our informal common use, rarely do we mention, or even notice this difference)
Anyway, when using the dash in english as well as Russian, you are essentially equating a subject with a definition. (Subject — definition) Most commonly in english, this is found in the dictionary (stick — a thin tree branch that has been separated from its mother plant) or it is also used when sourcing ("Two roads diverged in a yellow wood" — Robert Frost).
A hyphen on the other hand is supposed to join two words together (Latter-Day Saints) (sorry if you do not appreciate the reference, it was the first thing to come to mind) and originally, I believe, it was created in case you run out of space for a word on a page: (the dog was ru- nning up the hill)
Now in english, we use articles to fill in for the dash, making it less commonly necessary. (Tim is an actor) In Russian, these articles are not used, hence the dash is required. (Тим — актёр) But the purpose in constructing proper grammar remains the same.
Now for any native Russian speakers reading this, most English speakers ether forget, or don't really even learn this difference between hyphens and dashes, so you do not really need to worry too much about this, I put it here more or less as a reminder.
Now back to the original question ...
"Ребёнок — мальчик." actually means "the child is a boy."
Remember: subject first, then the definition you are equating it to.
So in the case of this duolingo statement:
Мальчик — этот ребёнок.
You are saying "A boy — this is a child" (much like a dictionary definition) because remember этот is referring to the subject, which is "a boy" not to the definition which is "a child"
And as for the difference between "The boy is a child" vs. "A boy is a child" you have to once again remember that Russian does not have articles, and both "a" and "the" are articles, the difference being that "the" implies this particular thing, whereas "a" implies anything that falls under the description of this word.
The need to distinguish between "this particular thing (or noun) in question" vs. "any thing (or noun) that falls into this category" is the sole purpose of the word это (or этот, эта, et cetera)
This was a lot of description, but with this knowledge, we can now understand this:
Этот мальчик — ребёнок (This boy is a child) (Would use this if there were several specific individuals who could be referred to as "the boy")
Мальчик — ребёнок (The boy is a child) (The same meaning as the last, but less specific as to which boy, likely meaning there is only one boy in question)
Мальчик этот ребёнок (The boy is this child) (Subject changes to become the word "child", not the word "boy")
Мальчик — этот ребёнок (A boy is a child)
"X - это Y" is a definition-like sentence, so you don't use it to describe a specific boy. It is like "A mouse is an animal", "Physics is a science", etc.
The boy is a child = Этот мальчик - ребёнок. Dropping "этот" would sound unnatural here. You can also say "Мальчик ещё ребёнок", like "The boy is still a child".
ы is i.
ь makes the consonant that is directly before it softer.
ш is sh as in shop or shoe.
ч is ch as in cheese or chair.
щ is shch sh+ch (ш+ч) together quickly, sounds like a hard SH
Here's a video explaining the Ukrainian alphabet which is pretty much the same in Russian. https://www.youtube.com/watch?v=ivOFPI5arwo