Learn Russian: Это гений, который изобрёл прибор в девятнадцатом веке. - This is the genius who invented the device in the nineteenth century.

The word который (who/which) connects the two parts of the sentence. It must match the gender and number of the noun it refers back to. Since it refers to гений (genius), which is a masculine singular noun, который is also masculine singular.

Why is который in the Nominative case?

Even though который gets its gender and number from the noun it points back to (гений), its case depends on its job in its own part of the sentence. Here, the "who" is the subject doing the action of inventing, so it stays in the Nominative case.

Is the comma before который required?

Yes! In English, we sometimes drop the comma or even the word "who" or "that". In Russian, you must always keep the word который and you must always put a comma before the clause that starts with it.

The device is the direct object of "invented", so why doesn't прибор change its ending?

It is actually in the Accusative case here! But remember that for inanimate masculine nouns (objects, not people or animals), the Accusative case looks exactly like the Nominative case dictionary form. So прибор doesn't change its spelling.

Why are there -ом and -е endings in в девятнадцатом веке?