Vehicles in Swedish. Also in singular and plural (indefinite) form.
People who are learning Swedish, very often use the singular form in a wrong way.
When to use "en" or "ett" is something you must learn by experience.
You can compare it with the english "a" or "an".
It is also good to know that "en" or "ett" is the same as "one" (like in one vehicle, one car, etc).