When your roast beef is so tough it's often because it was cooked at too high of a temperature or not cooked long enough.
Overcooking the roast beef can also make the roast beef turn out tough.
Overcooking the roast beef at high temperatures can cause the roast beefs muscle fibers to contact and squeeze out moisture and make the meat dry and chewy.
If it's a good cut of roast beef, then the roast beef being so tough often means you cooked it too long or at too high of a temperature.
If the roast beef started as a tough cut of beef, it means you didn't cook it long enough at a low temperature.
For tough cuts of roast beef cooked relatively quickly, you always have to cut it thinly against the grain
A good way to soften the roast beef is to marinate the roast beef in grated onion.
You can also use the onions for dipping in the sauce as they are and the roast beef will taste even better.
Marinate the roast beef in around 200 g of grated onion for 30 minutes to 2 hours.
After marinating the roast beef, wipe the onion and season it with some salt and pepper.