Make returning easy, because everybody does
Resumption is a core feature, not a convenience.
Almost every learner in a self-paced course leaves and returns, often after longer than the designer imagined. What they need on returning is to know where they are, what they last did, and what comes next, and most courses supply none of it beyond a progress bar.
A short orientation at the start of each session costs thirty seconds and answers all three: here is what you covered, here is what this segment does. It feels redundant to a designer reading the course straight through and it is the difference between resuming and rereading.
The technical side matters too. Progress must be saved reliably and frequently, and it must survive a closed browser rather than only a proper exit, because closing the tab is how people actually leave. Testing this specifically, by closing a session the way a real learner would, is a five-minute check that is skipped almost universally.
Where a course spans months, expect gaps of weeks and design for them. A summary page the learner can reach at any point, showing the shape of the whole course and their position in it, is more useful than any amount of encouragement to keep momentum.
The other element worth building for return is a way back to the specific thing the learner was doing rather than to the start of a section. Platforms typically resume at a page boundary, which can be some distance from where attention stopped, and a short note in the interface saying what the last completed activity was closes the gap.