I'm currently going through the Lernu course. In lesson 10 there are two sentences on the same page:
Mi parolis kun li tiam, kiam li venis al ni.
Mi tre ŝatas, kiam vi ridetas al mi.
Why tiam, kiam in the first sentence and not in the second? How do I know when to include tiam?
I guess the same thing could be asked about tio(n), kio(n), etc...