"I do not know if he is home."
Translation:Ik weet niet of hij thuis is.
The English word "if" is used in two different ways. Firstly, it can be used to mean "whether", like in this sentence. You are right, "if" and "whether" are completely interchangeable here. That is when Dutch uses the word "of".
Secondly, it can be found at the start of conditional subclauses. You could replace it with "provided that" or "in the case that" or sometimes just with "when", but not with "whether". In that case, Dutch uses "als". For example:
- If it rains, I do not go outside. - Als het regent, ga ik niet naar buiten.
- I go if you are going too. - Ik ga als jij ook gaat.