Historical Linguistics and Language Families 1, Exercise 2: sing / sang / sung

Synchronic view (English now): sing, sang, sung look irregular. A learner
just has to memorise the vowel change, as with other "strong" verbs.

Diachronic view (through time): the pattern reflects a once-regular
Indo-European vowel-change system (Wikipedia, Historical linguistics). What
looks like an exception today is the leftover of an old rule.

Lesson: the diachronic view explains oddities that the synchronic view can
only list.