Translation:This is a cheese knife; this is a fish knife.
Strictly from the perspective of a native English speaker, "this X is for" and "this is an X for" are wholly interchangeable and often incomprehensible from each other; the answer is the same, with different phrasing, regardless the varying questions that we're answering. What, then, are the actual rules between "это" and "этот", and how do we know when to use which?
@Mantrid_Brizon - "This" and "that" can be demonstrative pronouns or demonstrative adjectives and it changes the subject of the sentence.
In Russian, the pronoun is simply это (this) or то (that), but the adjectives are этот/эта/это/эти and то/та/те (plus they decline to match the gender, number and case of the noun they describe).
Here's a simple article about it, and I don't mean to insult anyone's intelligence but I also had a difficult time coming up with a simple answer as to the difference between them. https://7esl.com/demonstrative-pronouns-this-that-these-those/
I think one of the difficulties I had in answering was getting hung up on the rather specific example you gave, but these pronouns/adjectives are used in a variety of different contexts where they are not interchangeable.
Use the pronoun form both in English and Russian when literally or figuratively pointing to something so as to identify it.
Use the demonstrative adjective form when describing something that is already understood.
This (large object that looks exactly like a Ferrari) is actually a steam powered, high production, automatic knife. Don't get too close to it as it might activate automatically and suck you in.
This knife (which everybody understands you are talking about because it is clearly a knife and you are all looking at it) has poison in its wooden handle so always wear gloves in case you have a scratch or open wound. That knife (which is right beside the first knife) without the wooden handle is o.k.
It makes no difference which form you use when it makes no difference. But there are circumstances when it does make a difference.
There are periods in history when women routinely carried hand held fans to cool themselves. For fashion reasons, sometimes they were quite elaborate. Some Samurai women had sharp edges on their fans so that they could slash the jugular of an attacker.
If warning someone about an armed woman, you would not say.....grab the woman with that knife..... leaving everyone to ignore all the women with fans while looking for a belts or something that would hold a knife.
You yourself would say.....That is a knife (pointing at the fan) grab her and get that fan away from her.
First get clear about demonstrative pronouns and demonstrative adjectives in English whether you use them routinely or not. Then do the same with Russian which will be much easier when you have the English down pat. Then you are ready to learn the rules about how they are applied in Russian.
Conversely, you could just forget the grammar naming system for parts of speech and their rules. You could just think of co-locations. Some words just go together for some reason and some words never do for some reason.
In that system, you learn phrases instead of words, read short stories and repeat them out loud, watch movies etc and immerse yourself in the language as much as possible.
But Duo isn't the right platform for that. It is all about translation exercises. They expose you to the vocabulary and grammar at the same time and test you on it. That forces you to examine your understanding of the rules of your own language.
Duo is here, it is free and it is all laid out for you. Other approaches are less conventional and lack structure unless you pay for someone to build the structure for you like with a premium option.
Good ones are hard to find and success is hard to estimate.
Для seems to have different sounds dlia and dle. Is it right? Any rule?
Well, according to "The Phonetics of Russian" (Jones Ward, 2011), it depends on the context. If the я is followed by a palatalised (soft) consonant, it's probably going to be expressed as [jæ]. And in very rapid speech, the vowel could sound even higher (close to [jε]).
Native speakers (of Russian) would be the last people to realise this – that's only natural! But just compare the /a/ sound in сядь (sit!) to the one in сад (garden).
In this example, there is actually a slight difference in vowel quality, (probably due to the anticipation of the devoicing of the s in сыра, to be really pedantic). But I imagine Guido's question was more general, and the answer is yes, it does vary a little with the context.
I put "this is cheese knife; this is a knife for the fish" and the correct solutions were: "This is the knife for cheese; it's a knife for the fish." and " This is a cheese knife; this is a fish knife." Shouldn't mine also be accepted since it's basically a mix of the two?
Technically both are wrong. Well, yours is changing это's part of speech in the translation, DuoLingo's is just weird because you typically wouldn't use "the" if describing something's general function.
"This knife" would be этот нож, but here it just says "это нож", meaning "it is" or "this is" a knife.
Given that there is no punctuation in this app, it seems ridiculous the amount of sentenced that require hyphens or semicolons. This sentence doesn't even make sense to use a semicolon - the word 'and' is especially designed for sentences like this, maybe the duolingo developers could look this up in a dictionary.
.......This sentence doesn't even make sense to use a semicolon - the word 'and' is especially designed for sentences like this,.........
Sometimes people use different phrasing and sentence construction than you do. Using and in this sentence leads to a connection of some kind between the two items. But if the speakers point is that they are not connected but completely separate and unrelated items, he may want to avoid using and.
The developers of Duo often want to get across the point that there are many different ways of speaking and writing in English. This example shows one method of communicating a particular point.
I may choose to avoid using and when holding up a picture of two seemingly identical apples. I might well say......this one is a bomb; this one is an apple..... That is because I want to emphasize the difference. They are identical in every way except for one key aspect. That key aspect makes them entirely different for my purposes.