ExtensionArrayOfString.swift

extension BidirectionalCollection where Iterator.Element == String, SubSequence.Iterator.Element == String {
    var sentence: String {
        guard let last = last else { return "" }
        return count <= 2 ? joined(separator: " and ") :
            dropLast().joined(separator: ", ") + " and " + last
    }
}

let elements = ["a", "b", "c", "d", "e"]

let sentenceFromElements = elements.sentence // "a, b, c, d and e"