Realis is used
when reality strikes. The probability that the condition
set will materialise is quite high or better is considered
high by the one speaking.
If he runs 20 miles his feet
If you read a lot you are wise.
In English both sentences are in
indicativo (the reality mode), using the present tense
or the future. In Spanish both sentences are in present
indicativo. They just simply state a fact using
the si to indicate that there is a slight uncertainty.
Still, this uncertainty is not big enough to use subjuntivo
Si le da el
dinero, le deja en paz.
If he gives
her money she will leave him in peace.