Learn Farsi: این تمرین آسان نیست.

Yes, they are often interchangeable. آسان means "easy" (the opposite of hard), while ساده means "simple" (the opposite of complicated). For an exercise, both work perfectly.

Does تمرین refer to a physical workout or a homework exercise?

It can be both! Just like the English word "exercise," تمرین is used for academic practice (like a math or grammar drill) as well as physical training or sports practice.

How would this sound in everyday spoken Farsi?

It would sound almost identical, but native speakers usually drop the final "t" in نیست, pronouncing it as نیس. So you will often hear: این تمرین آسان نیس.

Why is نیست a separate word instead of using a negative prefix?

While most Farsi verbs are made negative by attaching the prefix نـ (like نمی‌دانم for "I don't know"), the "to be" verb is an exception. نیست is a standalone irregular word that serves as the negative counterpart to است (is).