one of my teachers says that ukrainian is an artificial language that was made in the nineteenth century by mixing polish and belarussian together.
to what degree is this true?

one of my teachers says that ukrainian is an artificial language that was made in the nineteenth century by mixing polish and belarussian together.
to what degree is this true?
To no degree. Rule of thumb: "If something sounds like nationalistic crap, it probably is."
More realistically, Ukrainian diverged from Russian sometime between the 14th and 16th centuries. If you want a more detailed analysis of the claims and counter-claims the relevant Wikipedia article is a fine place to start. A teaser: "By the mid 17th century, the linguistic divergence between the Ukrainian and Russian languages was so acute that there was a need for translators during negotiations for the Treaty of Pereyaslav, between Bohdan Khmelnytsky, head of the Zaporozhian Host, and the Russian state."

maybe he is on about the written language?
it's interesting to have teachers from totally opposed camps; like one who states that kievan rus was pure ukrainian not early russia, and then this guy.

I wonder what he means by 'artificial language'? Obviously languages develop from other languages - you could argue about what came first, or what had a greater influence over what, but the idea of it being artificial is just bizarre! It suggests that someone deliberately invented it (which may apply to some extent to the writing and grammar rules, but that goes for all languages and it just a reflection of what is in use at the time).

someone deliberately invented it (which may apply to some extent to the writing and grammar rules, but that goes for all languages
i like that argument.

I have this imagine now of some 'official Ukrainian language commission' travelling round Ukraine on horseback teaching the peasants how to talk ;-)
I wonder what he means by 'artificial language'? Obviously languages develop from other languages
There are languages that are without dispute artificially created: Esperanto and such. They are artificial in the sense that someone sat down, invented the words and rules, and thus artificially created the language. Originally, the language had no native speakers.
Of course it is possible that subsequently groups of Esperanto speakers came across concepts for which their language did not have a word yet, and that from there on the language grew and developed more naturally (for lack of a better word; I mean "more like other languages"), but originally it was an artificial language.
Perhaps mathilda's professor believes that Ukrainian was similarly developed by some committee somewhere who assembled it from Polish and Belarussian and then made it the language of instruction in schools.
nonsense.
Ukrainian is part of a dialect continuum that blends into Russian on the Eastern side and Slovak and Polish on the Western side.
As a student of language you will be aware of the effect of political organization on the formation of a standardized language. Once there were many different, but closely related, dialects spoken across France. Because Paris became the political center, standard French is based on the dialect that was spoken around Paris. Now hardly anybody can speak the local dialects. The same is true for most other European countries.
Ukraine didn't have a strong political center as it was usually on the periphery of the Russian (and Austrian) empires (the U Krajina- on the border). So although the local population spoke a dialect that was clearly different from Russian, there wasn't a real standard language.
In the 19th century there was a strong national revival movement across Europe that led to the standardization of many languages, and the use of 'peasant languages' for literary purposes. A similar thing happened in Slovakia where there was no agreed standard Slovak language as Slovakia had always been a backwater of Hungary. It was during this period that a 'standard' form of Ukrainian was defined. Taras Shevchenko was one of teh main proponents.
However, the fact that literary Ukrainian was formalized in the 19th century doesn't mean the language was 'invented'. Its roots are as old as other Slavic languages and the idea that Russian or Belarussian existed first, and that Ukrainian is a corupted or debased version of it is plain nonsense.