Questions & Answers about Dobra drenaža čuva korijen biljke.
Drenaža is in the nominative singular because it is the subject of the sentence — the thing doing the action.
In Dobra drenaža čuva korijen biljke, the drainage is what protects/preserves the root.
So:
- dobra drenaža = subject, nominative
- čuva = protects / preserves
- korijen biljke = object phrase