Questions & Answers about Nie widzieliśmy tego budynku.
Why is it widzieliśmy with an "e" instead of an "a" like in widziałem?
The verb widzieć (to see) has a stem that changes between -a- and -e- in the past tense. You use the -a- stem for most forms (like widziałem, widziałaś). However, for the masculine personal "we" and "they" forms (when referring to groups with at least one male), the stem changes to -e-, making it widzieliśmy (we saw) and widzieli (they saw).
Why is it tego budynku and not ten budynek?
Normally, the direct object of "to see" takes the Accusative case (as in Widzieliśmy ten budynek). But in Polish, whenever a sentence with a direct object is negated by nie, the object switches to the Genitive case. Since budynek (building) is a masculine inanimate noun, its Genitive form is budynku, and the word ten (this) changes to tego.
If a group of women were speaking, how would this sentence change?
If a group of women (or any group without men) were speaking, they would use the non-masculine personal form of the verb. For the verb widzieć, this form goes back to using the -a- stem. So, "we saw" would be widziałyśmy, and the whole sentence would be Nie widziałyśmy tego budynku.
Why does the genitive of budynek end in -u instead of -a?