I have a question about substitution in Swift's if statement.

For iflet n=maybe, is the variable maybe substituted for the variable n?

let, so n is a constant, not a variable, but it is correct to understand that you have substituted it.

This syntax is called Optional Binding and is characterized by not executing the then clause if the substitution result is nil.

 varmaybe: Int?=nil
if let n = maybe {
  n+1
  // not doing
}