MySQL Using a trigger to update quantity on another table if part is used
You need to have DELIMITER directives both before and after the CREATE TRIGGER statement, you need to end individual simple statements with semicolons, and you need to end the complete CREATE TRIGGER with the delimiter specified in the first DELIMITER directive:

DELIMITER //

CREATE TRIGGER subtract_quantity
AFTER INSERT ON parts_used FOR EACH ROW
BEGIN
  UPDATE parts
    SET parts.quantity = parts.quantity - 1
    WHERE parts.idParts = NEW.idParts;
END
//

DELIMITER ;


However, in this case your entire trigger body is a single UPDATE statement, so you can omit both the BEGIN ... END statements and the DELIMITER directives:

CREATE TRIGGER subtract_quantity
AFTER INSERT ON parts_used FOR EACH ROW
  UPDATE parts
    SET parts.quantity = parts.quantity - 1
    WHERE parts.idParts = NEW.idParts;


(The DELIMITER directives allow the parser to distinguish between the end of a compound-statement trigger or procedure body and the ends of the individual simple statements it contains, and are only required by some MySQL clients.)