Dad will reply to the letter when he comes home from work.
Dad will reply to the letter if he comes home from work.
The difference lies in the likeliness.
When implies high likeliness or certainty, which means the dad will definitely come home from work. If implies a lower level of certainty about that.
How about other time conjunctions? I feel like they convey high certainty. In other words, they are highly likely to happen.
We will do the shopping before they arrive.
As soon as he finishes collecting the data, I will start working on making the slides.
She will go jogging after it stops raining.
I will not go home until the construction work is done.