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.

In the first photo you can see the bird in the tree sitting next to a set of car keys.
I only saw those keys when I enlarged the photo.
Maybe someone has hidden the keys, but it could also be that a bird, such as an Eurasian Magpie, has put them there.

Hayarkon Park, Tel Aviv, Israel, 29 December 2019