Question

Change into passive voice - Tim had decided not to study further. Please reply quickly.

Answer 1

Before solving this question, we need to recall the following changes when a sentence is converted from active to passive voice.

Firstly, the object of the sentence becomes the subject and vice versa.
Example -cat killed the mouse.

Here the cat kills what? the mouse - this is our object and a "by" is added before the subject in passive voice.

Secondly, there are some tense changes 

Active  Voice                                            Passive Voice
Simple-
            Present                                          is (Participle form of verb)
            Past                                               was (Participle)
            Future                                            will be (participle)

Continuous
            Present                                          is being (Participle)
            Past                                               was being(Participle)

Perfect
            Present                                          have been (Participle)
            Past                                               had been (Participle)
 
There are other changes too but we will keep ourselves within this sentence only:Tim had decided not to study further.
Here , the subject is "Tim"
So on interchanging it with object, we get
........................ by Tim
Tense in active voice - Past perfect
in passive voice it will be ( had been (participle form of verb))
That is " had been decided"
So on completing the final answer we get:
Not to study further had been decided by Tim.

Answer 2

Not to study further has been decided by Tim.