***** NOT A TEACHER *****
Good afternoon Manish Birdi.
(1) I believe that in modern English, it is preferable to say:
I HAVE decided to study English.
I HAVE decided that I need more time.
Yes, sir, I HAVE decided. (I have made a decision.)
(a) Nowadays, "have" is usually used with the past participle to
form the present perfect.
(2) Perhaps a judge might say:
It IS hereby decided that ....
(a) But in modern English, people usually use a form of "have," NOT "be":
It HAS been decided that ....
Have a nice day!