Breakdown of Человек, который изобрёл этот самолёт — гений.
человек
person
этот
this [f: эта, n: это]
самолёт
airplane
который
which / who / that (relative pronoun)
Questions & Answers about Человек, который изобрёл этот самолёт — гений.
Why is который in the masculine, singular, Nominative form here?
Который (which/who) always matches the noun it refers back to in gender and number. Here, it refers to человек (person/man), which is masculine singular. It is in the Nominative case because it acts as the subject of the action inside its own little clause (it is the "who" doing the inventing).
Is the comma before который really necessary?
Yes! Unlike English, where commas with "who" or "which" depend on the type of clause, Russian always requires a comma right before который to separate the extra information from the main sentence.
What is the dictionary form (infinitive) of the verb изобрёл?
The infinitive is изобрести (to invent). It belongs to a group of verbs ending in -сти that have slightly irregular past tense forms. Because the subject of this clause (который) refers to the masculine noun человек, the past tense takes the masculine form изобрёл.
Why doesn't the ending of этот самолёт change if it's the object being invented?
It actually is in the Accusative case because it is the direct object of the verb. However, for masculine inanimate nouns (lifeless objects, rather than people or animals), the Accusative form is identical to the Nominative (dictionary) form. So, самолёт and its modifier этот look like they haven't changed.