Here's a breakdown:
* navis - the nominative singular form of the noun, meaning "ship".
* navem - the accusative singular form of the noun, used as the direct object of a verb.
For example:
* Videt navem. (He sees the ship.)
If you want to use "ship" as the subject of a sentence, you would use the nominative form "navis."
* Navis magna est. (The ship is large.)