Tips for searching:
Search field means any element you can search by (author, title, year, publisher, etc.). If you select "All" it means you will search in any field.
If you search for a combination of fields and words (phrases) you have to select a boolean operator between them (AND, OR, AND NOT). By default it is AND.
"Exact word" checkbox is not selected by default, which means you are searching for the word you entered and words similar to it, i.e. if you searched Екология - the results could contain Екология, Екологията. If you search for Еко - the results could contain Екология, Екологията, Екологични, Екологичните. If you check "Exact word" you will limit your search to ONLY the exact spelling you wrote - Екология.