There are often synonyms, that are not accepted as the correct answer
E.g. Samstag == Sonnabend, which is not accepted, although it is synonym. I have noticed similar problems also when using the French course.
Ist mir total Wurst, was Google ngram viewer anzeigt und wie oft. Es ist nur eine SW und kein Mensch. Ich lebe schon einege Jahre in Österreich also ich habe aus Erfahrung geschrieben. Und Rechner wird einfach viel mehr benutzt als Computer (vielleicht dort, wo du wohnst, ist es anders - kann sein). (ähnlich wie ordinateur statt computer im Französishen) Und ja es gibt sehr leichte Unterschiede zw. hier und da, aber in den meisten Kontexten sind die 2 Wörter vom Sinn her Synonyme.
@Lubos-Dobrovoda I agree that the synonym recognition could use some work. But it's not always that easy.
You're right in the "Samstag" == "Sonnabend" case. But "hier" and "da" aren't really interchangeable. "hier" refers to where the action takes place, like "here" in English. "da" has a more demonstrative meaning, it refers to wherever the speaker wants it to point. That can be here, it can be there, it can be anywhere
As far as "Rechner" and "Computer" are concerned, if anything, "Computer" is used far more frequently these days than "Rechner". Just as an example, the Google ngram viewer shows that "Computer" is used about 3 to 4 times as often as "Rechner". That's also about congruent to my experience as a native speaker.
Additionally, in German "Rechner" refers to anything or anybody that computes. That can be a computer, but it can also be, for example, a "Tarifrechner" on an insurance company's website. The English term "computer" includes that broader meaning as well, but the German loan word doesn't. So in German, "Rechner" and "Computer" aren't exactly synonymes.
Rechner is used much more than Computer. I think your tool is not really useful if one wants to learn German. I speak it at least at C1 level, as I am living in Austria for more than 5 years now and I am working here more than 9 years. You ve got a lot of work to be done.
There are also others like: hier = da, but it is not accepted. And these are very basic and fundamental.