If they are not singing, Common Chiffchaffs are very hard to distinguish from Willow Warblers (in Dutch: Fitis).
Chiffchaffs have dark legs and feet whereas Willow Warblers have pale legs and feet, although this is not always visible.
The Willow Warbler's supercilium over the eye is also generally stronger and more noticeable than that of the Chiffchaff.
But the main difference is the song: the Chiffchaff has a diagnostic repeating "chiff - chaff" call which is unmistakable whereas the Willow Warbler has a more melodic downward spiralling song with a little kick at the end.

Soomaa National Park, Estonia, 23 April 2017