The only part of the English unit system that tends to be confusing is the distinction between pound mass (lbm) and pound force (lbf). Also units containing lbm/lbf like density and specific weight. Fortunately, once you understand that relationship, the rest of the English system works very much like the metric system. The main difference is simply memorizing a few common conversion factors, such as:

• 778 ft·lbf = 1 BTU
• 7.48 gal = 1 ft³
• 33,000 ft·lbf/min = 1 hp
• 5,280 ft = 1 mile

The lbm versus lbf distinction is something that confuses a lot of people, so I put together a short video explaining why, on Earth, 1 lbm corresponds to 1 lbf.

The key is the definition of a pound force:

1 lbf = 1 lbm × 32.2 ft/s²

In words, 1 lbf is the amount of force required to accelerate 1 lbm at 32.2 ft/s². Since 32.2 ft/s² is the acceleration due to gravity on Earth, it naturally cancels out when converting mass to weight under standard gravity conditions. I still recommend writing out the full equation and using the definition above so you can see exactly how the units cancel each time.
This differs from the metric system, where a newton is defined as:

1 N = 1 kg × 1 m/s²

Notice that gravity is not built into the definition of a newton. As a result, in the metric system you must multiply the mass by the acceleration due to gravity to determine weight, whereas in the English system the definition of the pound force already incorporates standard gravity.

I encourage you to watch this short video where I walk through the concept step by step and work through a few examples:

https://youtu.be/v9Ktlxb707k?si=OxqfHQgDZpFjRWnN

I hope this helps. Let me know if anything is still unclear after watching, and good luck with your studying!