user18528: This is the scenario: I pick up my Knight to capture my opponent's Bishop (touching the Bishop). I then realise my Knight was pinned to my King in the first place, so moving it would be illegal. However, I have already touched my opponent's Bishop with my Knight. Since I have touched my opponent's Bishop, would I have to capture the Bishop with another piece of mine (in this case my Queen can legally capture the Bishop instead), or am I allowed to just move any other piece entirely? Reference in FIDE Rules is Article 4.3.3 but it still seems it doesn't explain this scenario well enough.

user17146: The relevant sections from https://www.fide.com/fide/handbook.html?id=208&view=article:

  4.2.1
  
  Only the player having the move may adjust one or more pieces ...
  
  4.2.2
  
  Any other physical contact with a piece, except for clearly accidental contact, shall be considered to be intent.
  
  4.3
  
  Except as provided in Article 4.2, if the player having the move touches on the chessboard,with the intention of moving or capturing:
  
  4.3.1
  
  one or more of his own pieces, he must move the first piece touched that can be moved
  
  4.3.2
  
  one or more of his opponent’s pieces, he must capture the first piece touched that can be captured
  
  4.3.3
  
  one or more pieces of each colour, he must capture the first touched opponent’s piece with his first touched piece or, if this is illegal, move or capture the first piece touched that can be moved or captured. If it is unclear whether the player’s own piece or his opponent’s was touched first, the player’s own piece shall be considered to have been touched before his opponent’s.

Article 4.2.2 establishes that any non-accidental contact shall be considered intent, so touching the opponent's bishop with your knight establishes intent to capture. At that stage, you've touched both your knight and your opponent's bishop - that is, you've touched "one or more pieces of each colour" -  so article 4.3.3 dictates what happens next.
Let's break down article 4.3.3, then. It first says you must:

capture the first touched opponent’s piece with his first touched piece

(nope, you can't do that - it's an illegal move)

or, if this is illegal

(it is)

move or capture the first piece touched that can be moved or captured

Since your knight cannot be moved, the "first piece touched that can be moved or captured" is your opponent's bishop. Therefore you must capture it.

user4418: Article 4.3.2 clarifies this -

4.3 Except as provided in Article 4.2 [j'adoube], if the player having the move touches on the chessboard,with the intention of moving or capturing:
4.3.2 one or more of his opponent’s pieces, he must capture the first piece touched that can be captured

In the scenario you describe you touch your knight and your opponent's bishop with the intention of capturing the bishop with your knight. Moving the knight is illegal hence you may not capture the bishop with the knight. However capturing the bishop with some other piece, your queen, is legal. Therefore you must capture the bishop with that piece.

18528: <p>This is the scenario: I pick up my Knight to capture my opponent's Bishop (touching the Bishop). I then realise my Knight was pinned to my King in the first place, so moving it would be illegal. However, I have already touched my opponent's Bishop with my Knight. Since I have touched my opponent's Bishop, would I have to capture the Bishop with another piece of mine (in this case my Queen can legally capture the Bishop instead), or am I allowed to just move any other piece entirely? Reference in FIDE Rules is Article 4.3.3 but it still seems it doesn't explain this scenario well enough.</p>
 17146: <p>The relevant sections from <a href="https://www.fide.com/fide/handbook.html?id=208&amp;view=article" rel="nofollow noreferrer">https://www.fide.com/fide/handbook.html?id=208&amp;view=article</a>:</p>

<blockquote>
  4.2.1
  
  <p>Only the player having the move may adjust one or more pieces ...</p>
  
  4.2.2
  
  <p>Any other physical contact with a piece, except for clearly accidental contact, shall be considered to be intent.</p>
  
  4.3
  
  <p>Except as provided in Article 4.2, if the player having the move touches on the chessboard,with the intention of moving or capturing:</p>
  
  4.3.1
  
  <p>one or more of his own pieces, he must move the first piece touched that can be moved</p>
  
  4.3.2
  
  <p>one or more of his opponent’s pieces, he must capture the first piece touched that can be captured</p>
  
  4.3.3
  
  <p>one or more pieces of each colour, he must capture the first touched opponent’s piece with his first touched piece or, if this is illegal, move or capture the first piece touched that can be moved or captured. If it is unclear whether the player’s own piece or his opponent’s was touched first, the player’s own piece shall be considered to have been touched before his opponent’s.</p>
</blockquote>

<p>Article 4.2.2 establishes that any non-accidental contact shall be considered intent, so touching the opponent's bishop with your knight establishes intent to capture. At that stage, you've touched both your knight and your opponent's bishop - that is, you've touched <em>"one or more pieces of each colour"</em> -  so article 4.3.3 dictates what happens next.</p>

<p>Let's break down article 4.3.3, then. It first says you must:</p>

<blockquote>
  <p>capture the first touched opponent’s piece with his first touched piece</p>
</blockquote>

<p>(nope, you can't do that - it's an illegal move)</p>

<blockquote>
  <p>or, if this is illegal</p>
</blockquote>

<p>(it is)</p>

<blockquote>
  <p>move or capture the first piece touched that can be moved or captured</p>
</blockquote>

<p>Since your knight cannot be moved, the "first piece touched that can be moved or captured" is your opponent's bishop. Therefore you must capture it.</p>
 4418: <p>Article 4.3.2 clarifies this -</p>

<blockquote>
  <p>4.3 Except as provided in Article 4.2 [j'adoube], if the player having the move touches on the chessboard,with the intention of moving or capturing:</p>
  
  <p>4.3.2 one or more of his opponent’s pieces, he must capture the first piece touched that can be captured</p>
</blockquote>

<p>In the scenario you describe you touch your knight and your opponent's bishop with the intention of capturing the bishop with your knight. Moving the knight is illegal hence you may not capture the bishop with the knight. However capturing the bishop with some other piece, your queen, is legal. Therefore you must capture the bishop with that piece.</p>
