user2429: Do FIDE rules allow a player to make notes on the scoresheet like 

Here, I wanted to play c4, but thought it would be better to play d4
  first.
He played Nc6. I think he should have played Nbd7.


user924: No. Notes could help you during the game, and therefore they aren't allowed. The Laws of Chess say:

12.3.a During play the players are forbidden to make use of any notes, sources of information or advice, or analyse on another chessboard
[...]
12.4 The scoresheet shall be used only for recording the moves, the times of the clocks, the offers of a draw, and matters relating to a claim and other relevant data.


2429: <p>Do FIDE rules allow a player to make notes on the scoresheet like </p>

<blockquote>
  <p>Here, I wanted to play c4, but thought it would be better to play d4
  first.</p>
  
  <p>He played Nc6. I think he should have played Nbd7.</p>
</blockquote>
 924: <p><strong>No</strong>. Notes could help you during the game, and therefore they aren't allowed. The <a href="http://www.fide.com/fide/handbook.html?id=124&amp;view=article" rel="noreferrer">Laws of Chess</a> say:</p>
<blockquote>
<p>12.3.a During play the players are forbidden to make use of any notes, sources of information or advice, or analyse on another chessboard
[...]</p>
<p>12.4 The scoresheet shall be used only for recording the moves, the times of the clocks, the offers of a draw, and matters relating to a claim and other relevant data.</p>
</blockquote>
