Going back over a lesson is always safe. Your record only moves forward, so review as often as you like.
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Open the Learn tab and find the lesson you’d like another look at. Completed lessons stay exactly where they were, ticked and tappable.
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Open it and work through it again, at whatever pace you like.
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Everything behaves as it did the first time: the explanations, the questions, everything.
Open this picture full size. Step 3: a lesson in review works exactly as it did first time. -
Leave whenever you like. Your record is untouched: the tick stays, the scores stay, and nothing is ever marked backwards.
In short: review anything, any time. Your record only moves forward.