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.

1st & 2nd photo: Huizen, The Netherlands, 6 April 2020
3rd photo: Bird banding site, Naarden, The Netherlands, 1 August 2010
4th photo: Oostvaardersplassen, The Netherlands, 7 October 2019
5th photo: Zouweboezem, Lexmond, The Netherlands, 15 April 2019