Breakdown of Mój dziadek zbudował ten dom w tysiąc dziewięćset czterdziestym roku.
dom
house
ten
this (m)
mój
my
w
in / on (a day)
Questions & Answers about Mój dziadek zbudował ten dom w tysiąc dziewięćset czterdziestym roku.
Why is only the last part of the year, czterdziestym, changed to match roku?
In Polish, years are read as a mix of regular (cardinal) numbers and ordinal (ranking) numbers. The thousands and hundreds stay as regular numbers that don't change: tysiąc dziewięćset (one thousand nine hundred). Only the tens and units are ordinal (like "fortieth"), and they must decline to match the case of the noun. So, "in 1940" is literally "in the one thousand nine hundred fortieth year".
What case is used for the phrase w ... czterdziestym roku?
This is the Locative case. When we say that something happened in a specific year, we use the preposition w followed by the Locative. The noun rok changes to roku, and the ordinal number changes to czterdziestym to match it.
Why doesn't the object ten dom change its ending here?
It actually is in the Accusative case because it is the direct object of the verb zbudował. However, for masculine inanimate nouns like dom (and their modifiers like ten), the Accusative form is exactly identical to the Nominative (dictionary) form. If it were a masculine animate noun, the ending would have changed.
Why use zbudował instead of budował for "built"?
zbudował is the perfective form of the verb, which emphasizes that the action was completely finished. The grandfather successfully finished building the house in 1940. If you used the imperfective budował, it would mean he "was building" it in 1940, focusing on the process without confirming if he ever finished it.