You have seen that Turkish uses the -mElİ suffix for indicating necessities and requirements,
and that it roughly translates into should and must depending on the context.
Since -mElİ can also be translated with should,
it can give the impression that the subject can also choose not to do what is said.
To express strong obligation, where the subject has no choice but to do what is said,
the words zorunda, şart and mecbur are used.
They correspond to have to in English.
Zorunda is used after infinitives and it's conjugated with personal markers.
I have to go to school.
Okula gitmek zorundayım.
You have to sign this document.
Bu belgeyi imzalamak zorundasınız.
Mecbur is used with light infinitive plus dative case suffix, and it is also followed by a personal marker.
Mecbur is an older version of zorunda.
It is used by elderly people or in poetic contexts and you should know it,
but if you would like to express an obligation yourself, using zorunda will sound more natural.
They have to kill me.
Beni öldürmeye mecburlar.
You have to obey the captain.
Kaptana itaat etmeye mecbursun.
Şart is similar to gerek and lazım, but it conveys obligation rather than necessity.
Like gerek and lazım, you don't add any personal suffixes after it.