More exercise in avoiding trouble. It’s early. White has moved a pawn and one of his bishops; Black has just made c7-c6 his second move. What should White play? The important thing to notice is that now the White d-pawn and Black c-pawn both have been moved. With Qa5+ the Black queen can check the White king and also attack anything on the fifth rank, such as White’s loose bishop. White should protect the bishop or block the queen’s path to the king with a move like c2-c3. If White instead plays e2-e3 here, naturally Black replies Qa5+, winning a piece.

Notice, by the way, that in the position as diagrammed Black would not quite be ready to win anything with Qa5; the move is a queen fork, but White has a solid response in Bd2 or Qd2, either of which both blocks the check from a5 and gives protection to the bishop. It is a regular part of considering a fork to ask whether both prongs of it can be blunted by a single move like one of these. But if White plays his pawn to e3 those defensive moves no longer work because he has obstructed the path from his bishop to the d2 square.

Study the visual appearance of the Black queen’s path here—the elbow-like pattern of open black squares leading from its position to the White king. It's something you never want to overlook. You also can treat this as a case study in the danger of leaving pieces on the board that have no protection but that seem safe because nothing is attacking them. They are prey to forks. The sight of White's bishop sitting out on g5 like this, bereft of protection, should make you edgy.