Translation:Matěj and František are good fathers.
Try this site to look up the possible forms. It's pre-filled for "otec", but the box next to the "Hledej" button can be used to enter the word you are looking for, and then click the button.
For words that have interesting declensions (like nouns), a table is displayed showing the standard forms across the two numbers (singular in the first column, plural in the second) and the seven cases (in as many rows).
The cases are designated by ordinal numbers. The ones you need for now are 1. nominative, 2. genitive, and 4. accusative.