Srila Prabhupada teaches us that one should follow the instructions strictly based on guru-sadhu-sastra (three together). The qualification of any instruction to be coming from Krsna is to be in line with guru-sadhu-sastra altogether. Srila Prabhupada also says that guru is one who speaks in line with sadhu and sastra. So one may not accept instruction of guru if it is not in line with sadhu and sastra.

But here there is a little confusion for me:

1. How can one know sadhu-sastra without a guru?
2. If one can know sadhu-sastra without guru then what is the need of a guru?
3. If guru adjusts something in sadhu-sastra to fit his ulterior motives how a disciple will know, because he will be explained the distorted meaning (interpretation) of sadhu-sastra?
4. If one's guru doesn't support the statement/order that is given by sadhu-sastra then should one follow that order or not, as the guru-sadhu-sastra (the trio) is not fulfilled?