to refill / fill in
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to refill
To make something that has become empty or partly empty full again. This can refer to liquids in containers, goods on shelves, or also data in a form.
Die Regale im Supermarkt werden über Nacht wieder aufgefüllt.
The shelves in the supermarket are restocked overnight.
'Auffüllen' is a separable verb. The prefix 'auf-' goes at the end in a main clause, but stays connected to the verb in a subordinate clause. In the infinitive with 'zu', 'zu' goes between the prefix and the verb.
Der Kellner füllt mein Glas wieder auf.
The waiter refills my glass.
The word consists of the prefix 'auf-' and the verb 'füllen' (to fill). The prefix 'auf-' reinforces the meaning here and emphasizes that something is filled again to a full state.
Bitte füll die Kaffeekanne wieder auf, wenn sie leer ist.
Please refill the coffee pot when it's empty.
If you want to specify what something is filled with, use the preposition 'mit' followed by the dative. The pattern is: etwas (Akkusativ) mit etwas (Dativ) auffüllen.
Er füllte die Flasche mit frischem Wasser auf.
He refilled the bottle with fresh water.
Common combinations are 'Vorräte auffüllen' (to replenish supplies), 'ein Glas auffüllen' (to refill a glass), 'den Tank auffüllen' (to fill up the tank), or also, in an abstract sense, 'ein Formular auffüllen' (to fill in a form).
Bevor wir in den Urlaub fahren, müssen wir das Auto volltanken und die Reiseapotheke auffüllen.
Before we go on vacation, we need to fill up the car and restock the travel first-aid kit.
to add / supplement
'ergänzen' and 'auffüllen' both have to do with something being missing and becoming fuller or more complete again. 'ergänzen' is often used with information, lists, texts, or missing parts. 'auffüllen' fits better when something is empty or partly empty and is made full again, for example a glass, a shelf, a form, or a hole with material. They can be confused because in both cases something is added. In practical terms: you usually use 'ergänzen' for information, while you use 'auffüllen' for containers, supplies, or gaps.
Vor dem Termin füllt sie die fehlenden Felder im Formular auf.
Before the appointment, she fills in the missing fields on the form.
Du ergänzt die Einkaufsliste, wenn dir noch etwas einfällt.
You add to the shopping list if you think of something else.