Questions & Answers about Tvar budovy je důležitý.
Because důležitý agrees with tvar, not with budovy.
The subject of the sentence is:
- tvar = shape
And tvar is a masculine inanimate noun. So the adjective must also be in the masculine singular form:
- tvar je důležitý = the shape is important
Even though budova is feminine, it is only part of the genitive phrase budovy (of the building), not the noun that the adjective describes.