Hi guys , I have an ectomorph body, I'm very strong and have great cardio and stamina, but I just can't gain weight, I lift 4 days a week, but have zero knowledge about nutrition, please advice what diet to follow to gain weight, thank you.
You don't eat enough. Plain and simple. Use a TDEE calculator and figure out how many calories you burn in a day. Then track what you eat and ensure you are at a 500-700 calorie surplus EVERY SINGLE DAY! It is all about consistency. If you don't do it every day then you will just spin your wheels.