Hi Sinem, There are two things worth pointing out about these sentences. 1. Tense. I'm sure that some native speakers would say it this way, but it probably isn't quite right. It would be better to say "If I allowED you to do it, then I'd have to allow everybody do it." Or If "I allow you to do it, ,then I'll have to allow everybody do it." The "let" sentence is fine in terms of tense, as "let" is also the past form of the verb "to let". 2. We "allow someone TO do something", but "let someone do something". i.e. there is no "to" with the verb "let", but there must be a "to" with allow. --> If I let you do it, then I'd have to let everybody do it. --> If I allowed you to do it, then I'd have to allow everybody to do it.