"Do you have a driving license to drive a truck?"
Translation:Kamyon sürmek için ehliyetin var mı?
For it to be "mısın", the subject has to be "sen" (you).
An example for verb to be with the subject as "sen":
Zengin = rich
Zenginsin = you are rich
Zengin misin? = Are you rich? (Here, you add the -sin, because you're really saying 'you are / are you')
An example for possessives:
Ehliyetin var = lit. your license is existant (you have a license)
Ehliyetin var mı? = Is your license existant? (do you have a license?)
An example where you would use "var mısın":
Takımda sen de var mısın? = Are you also existant in the team? (Are you also (listed) in the team?)
The subject is "you". (If it weren't a question, it would be: varsın)
because it refers to 'driver lıcence' and not to 'you' I don' t know if I use the preposition correctly as English is not my mother language, but it is like 'is there a licence for driving a truck at/on (?) you. ' If you say 'I have a drıver lıcence' you also say in Turkish something like 'My drıver licence exists'