To Decrease/Increase Prayer's Parts Unknowingly
If a person — as he/she does not know the rule —increases or decreases a part — other than a rukn — of prayer, their prayer is valid provided that they did not neglect learning the rule; otherwise their prayer is not valid by obligatory caution.
To Supplicate in English While Praying
In both mustaḥabb and obligatory prayers, there is no problem in supplicating in Persian or any other language whether or not it is in qunūt.