I have always understood that the basic rule is to make a new word (or words) using a letter (or letters) that are already on the board. I recently played a game in which my opponent did the following:-
From the word cake[vertically] they added an "S" to make "cakes" and then used the"S" [horizontally] to make another word---eg.story. ---surely that is having two turns. Am i correct?
Adding one or more letters to a word or letters already on the board.
Placing a word at right angles to a word already on the board. The new word must use one of the letters already on the board or must add a letter to it.
Placing a complete word parallel to a word already played so that adjacent letters also form complete words".
Cake was already on the board. He then added story after the e at right angles to make two words, cakes and story. Sounds OK to me, although I am not a Scrabble player.
That seems quite a basic acceptable scrabble move. The real experts, given the right circumstances, can add to 2 words at least and create a new one for 3 scores if the tiles fall right.