Beef or Lamb contains a lot of small grains. Once the beef is cooked, the grain is shrink which make it hard to eat. No matter how well you marinade it, without a proper cutting, it is hard to have it tender.
How to cut the beef correctly:
<< As you see the graphic on the left, the vertical white lines are the grain of the beef. To cut across the grain, you have to cut perpendicular as per the black lines (horizontally).
<< This is another example. The horizontal white lines are the grain . To cut across, you slice it vertically (the black lines)