Caution: "il" in the sentence you ask about does not mean impersonal "it", it means "he":

Il doit partir -- He has to leave.

You cannot say "il doit que...". If you want to make it impersonal, you have to use "falloir":

Il faut que nous partions. -- We must leave. (literally: it is necessary that we leave).

There is one construction in common use, which uses "devoir" in an impersonal way:

comme il se doit -- as it must be

Hi Karen,

https://www.lawlessfrench.com/subjunctivisor/

As I understand it the construction "Il doit que"....  does not require the subjunctive.

Thank you so much, 

And i've got an other question about subjonctif, can we conjugate these verbes: {penser, croire, imaginer, espérer} 

on "futur proche ou future simple"? 

I mean are they common? 

Ex: Après le procès de la semaine prochaine, Julie croira que Margaret est innocente.

And also:Après le procès de la semaine prochaine, Julie ne croira pas que Margaret soit innocente.

(Sry if it's confusing, Idk why this question just came to me)