Hi Sumesh,

It is a classic case of -

Si + present ------> future tense 

Si tu viens, on ira au parc = If you come, we will go to the park

S'ils arrivent en retard, tu me préviendras = If they arrive late, you'll let me know 

Although in the last example the order of the clauses in inverted which probably caused your confusion.

Hope this helps