Learn Polish: Ten wynalazek był wielkim przełomem w dwudziestym wieku. - This invention was a great breakthrough in the twentieth century.

To express "in the [ordinal number] century", Polish uses the preposition w followed by the Locative case. The basic dictionary form is dwudziesty wiek, but because it follows w to describe a period of time, the adjective changes to the Locative dwudziestym and the noun changes to the Locative wieku.

Why does the Locative form of wiek end in -u rather than -e?

While many masculine inanimate nouns take an -e ending in the Locative case, nouns ending in the consonants -k, -g, or -ch are exceptions. To avoid complicated spelling and pronunciation changes, they take the -u ending instead. Because wiek ends in -k, its Locative form is wieku.

Why does the sentence use wielkim przełomem instead of the dictionary form wielki przełom?

This is a refresher on the Instrumental case. When you use a form of the verb być (here as był, meaning was) to define what the subject is or was using a noun phrase, that noun phrase must take the Instrumental case. Therefore, the Nominative wielki przełom changes to the Instrumental wielkim przełomem.

Is there a logical connection between the Polish word przełom and the English word breakthrough?