
sich häufen
to pile up
To accumulate in large numbers or quantities; to become more and more numerous.
In letzter Zeit häufen sich die Beschwerden über den Lärm.
Recently, complaints about the noise have been piling up.
to pile up
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to pile up
To accumulate in large numbers or quantities; to become more and more numerous.
In letzter Zeit häufen sich die Beschwerden über den Lärm.
Recently, complaints about the noise have been piling up.

to pile up
To stack something into a pile or gather it together.
Der Gärtner häuft das Laub in einer Ecke des Gartens.
The gardener piles the leaves in a corner of the garden.
The verb 'häufen' (to pile up) is closely related to the noun 'der Haufen' (the pile, the heap). It describes the action of making something into a pile or of something becoming a pile.
Die Kinder häufen Sand zu einer großen Burg.
The children pile sand into a large castle.
There are two uses: 'sich häufen' (to pile up, reflexive) means that something increases on its own, often with abstract things. 'etwas häufen' (to pile something up, transitive) means that someone actively gathers something into a pile.
Die Fehler in dem Dokument häufen sich.
The errors in the document are piling up.
The reflexive form 'sich häufen' (to pile up) is usually used for abstract things that increase in number. Typical subjects are 'die Probleme' (the problems), 'die Fehler' (the errors), 'die Anfragen' (the inquiries), or 'die Beschwerden' (the complaints).
Seit der neuen Software häufen sich die Probleme im System.
Since the new software was introduced, problems in the system have been piling up.
to collect / gather
Sammeln refers to bringing several things together and can emphasize making a deliberate selection. Häufen, by contrast, means stacking things into a pile. When the result is meant to be a pile, häufen is the better choice.

Im Garten häuft Lina die trockenen Blätter neben dem Weg auf.
In the garden, Lina piles the dry leaves up beside the path.
Im Garten sammelt Lina die trockenen Blätter in einem Korb.
In the garden, Lina collects the dry leaves in a basket.