Irregular verbs
The bane of all non-native speaking French students! I took 4 years of French from 9th-12th grade, spent time in France, and after becoming fluent in Spanish later, I can say without reservation that French is NOT easy to learn to speak properly, let alone fluently! I was having problems with French irregular verbs (which most of them seem to be), so my mother who was pretty good with French put together a set of flash cards and started drilling me daily (10th grade as I recall). After a couple of weeks of this, I woke up one night in a sweat, having a nightmare - conjugating irregular French verbs in my sleep! Shudder...