Fu and hu
For the people who were born in the south of China, specially in Hunan province, not so many people pay attention to the difference between the pronunciations of fu and hu.
For example, when the south people pronounce 湖南(húnán), it sounds like funan or wunan. If you pay attention to the difference, it is easy to pronounce correctly.
F, your top teeth bite your bottom lip lightly.
W, Co-articulated Approximant,
Native Chinese always make the mistake. For it is easy for foreign people instead. For example, not so many native English speaker would mess up fire, hire and wire.