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 photo: Carrusqueira, Sado Estuary Nature Reserve, Portugal, 1 October 2021
2nd photo: EVOA Visitor Centre area, Tagus Estuary Nature Reserve, Portugal, 12 October 2021
3rd, 4th & 5th photo: Ria Formosa National Park, Algarve, Portugal, 18 & 20 November 2017