The blog David Hall mentioned in his comment to the question (this one) covers the
ob part very well, but your question differs slightly. So, we should shift the focus a little bit:
If you want to express a conditional like
If it is good wheather tomorrow, I will play football.
you cannot use
ob but have to use
Wenn morgen schönes Wetter ist, werde ich Fußball spielen.
If you want to express an uncertainty/several possibilities like
I don't know, whether I have time.
you should not use
wenn/falls but should use
Ich weiß nicht, ob ich Zeit habe.
Remark: You can use
wenn/falls in the second case, but you will mean something different, and in this case strange, thing:
Ich weiß nicht, wenn ich Zeit habe. = In case I have time, then I don't know (what to do).
This sentence is grammatically correct and might be suitable depending on context, but out of context it is missing the information, what
I don't know.