I have the same question that several other learners have asked. About the placement of "tard, plus tard, tot, plus tot, etc" at the beginning as well as the end of the sentence. I did the same thing as Michael. I had selected the option using "plus tard" at the beginning of the sentence, but then deleted it after reviewing the lesson.
Here is what the lesson states. (There is no mention of these adverbs being placed at the beginning of the sentence):
"Adverbs of place and certain adverbs of time usually FOLLOW the past participle:
e.g. tard, tôt,... and some adverbs ending in -ment
Il est parti tard. He left late.
Elle a compris facilement.She understood easily."
May I suggest that you give some examples with them at the beginning of a sentence? It would be very helpful.