Can it be predicted when a tornado will happen?

Only to a very limited degree. On the long term, analysis of large scale weather patterns can determine a period of a few hours in which tornadoes are likely to form in a region. However, this analysis cannot predict where or when individual tornadoes will strike.

On the sorter term, radar analysis can determine if a tornado is likely to produce a tornado in the next few minutes, though it still cannot say when a tornado will form.

If a tornado has already formed and we know its location, speed, and direction, we can gauge approximately when it will reach certain locations.