Why grammar should be explained in your language

The rule is the same. The learner is not.

Take one Serbian rule: after u and na, when you are saying where something is, the noun takes the locative. Muzej je na trgu, the museum is on the square.

For a Russian speaker this is barely a rule. Russian has the same case, the same two prepositions, the same use. The explanation they need is one line, and then a warning about the places where Serbian differs from what their instinct will produce. For a German speaker the idea is familiar, since German also changes the noun after a preposition, but the endings are new and there are seven cases where they know four. For an English speaker there is nothing to attach the rule to. English has no cases at all. The explanation has to start with what a case is, why the word changes, and why trg and trgu are the same word.

Three learners, one rule, three different lessons. A course that writes one explanation and translates it into three languages has written it for one of them and made the other two read a text that is either patronising or missing its first page.

Interference is specific to the pair

The mistakes a learner makes are not random. They are the shapes of their own language pushing through. A Russian speaker learning Serbian will meet hundreds of words that look identical and mean something else, and will use Russian word order for the little words, which Serbian places differently. An English speaker will drop endings, because English does not have them, and will reach for an article that does not exist. A Serbian speaker learning English will do the reverse: leave out the and a, because nothing in Serbian tells them one is needed, and will be understood and wrong for years.

None of these mistakes can be predicted from the target language alone. They come from the pair. An explanation that does not know where the reader is coming from cannot warn them about the one mistake they are about to make, and cannot skip the twenty they never would.

Translated is not adapted

Most courses are written once, in English or in the target language, and then translated. Translation preserves the sentences and loses the reader. The German version of an English course still explains cases to someone who already has them, and still says nothing about the false friends between German and the target, because the English original had no reason to mention them.

An adapted course is written from scratch for each native language: the same rule, the same facts, and a different explanation, with the comparisons that reader can use and the warnings that reader needs. It is more work, roughly one course per native language rather than one course translated, and it is the only way the explanation is actually about the reader.

What this looks like in the course

Every rule is explained in the learner's own language, written for speakers of that language rather than rendered from a generic text. A German and a Russian learner get different explanations of the same rule. The examples lean on what the reader already has: a Russian speaker sees where Serbian matches Russian and where it does not; an English speaker gets the concept built up before the ending is introduced.

The same holds at every level, not only at the start. The grammar of the last lesson is as adapted as the grammar of the first, because the interference does not stop when the learner gets better. It gets subtler.

And when the check marks a wrong form in a sentence you typed, the rule it shows you is that adapted explanation, in your language, at the moment you need it. Not a verdict, and not a page from a reference grammar written for everybody. The one thing you got wrong, explained to you.