abfallen
to fall off
To detach from something and fall down, as with leaves, fruit, or plaster.
Nach dem Sturm sind viele Äpfel vom Baum abgefallen.
After the storm, many apples fell off the tree.

to fall off / decline
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to fall off
To detach from something and fall down, as with leaves, fruit, or plaster.
Nach dem Sturm sind viele Äpfel vom Baum abgefallen.
After the storm, many apples fell off the tree.

'Abfallen' is a separable verb. In a main clause, the prefix 'ab' stands at the end. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Im Herbst fallen die Blätter von den Bäumen ab.
In autumn, the leaves fall off the trees.
The verb consists of the prefix 'ab-' (away, down) and the verb 'fallen' (to fall). The literal meaning 'herunterfallen' (to fall down) is the basis for all the other, often figurative meanings.
Vorsicht, der Putz von der Decke fällt ab!
Careful, the plaster is falling off the ceiling!
Since 'abfallen' describes a movement or a change of state, the perfect tense is always formed with the auxiliary verb 'sein' (to be).
Die Temperatur ist über Nacht stark abgefallen.
The temperature fell sharply overnight.
In the meaning 'abschüssig sein' (to be sloping), a direction is often given, for example with 'zu' (to), 'nach' (toward), or 'hin' (toward/down).
Die Küste fällt hier steil zum Meer hin ab.
The coast slopes steeply down toward the sea here.
This figurative meaning is used for abstract things. While physical objects (leaves, fruit) 'abfallen' (fall off, meaning 1), abstract values (performance, quality, fever) 'abfallen' (decline).
Die Qualität der Produkte ist in den letzten Jahren abgefallen.
The quality of the products has declined in recent years.
In the meaning 'sich abwenden' (to turn away), 'abfallen' is always used with the preposition 'von' and the dative.
Er ist von seinem Glauben abgefallen.
He has turned away from his faith.
In this meaning, it is often used colloquially to ask whether a part of something is left over for someone, e.g., a piece of cake or a small profit.
Wenn du einen großen Auftrag bekommst, fällt für mich dann auch etwas ab?
If you get a big contract, will there be something left over for me too?
to sink / fall
'sinken' and 'abfallen' can both describe performance, quality, value, or intensity becoming worse or lower. 'sinken' is neutral and works well with numbers, values, temperatures, or a general level. 'abfallen' puts more emphasis on a decline, often compared with before or with others. For example, you say that performance 'abfällt' when it becomes noticeably worse. Learners may confuse the words because both express downward movement in a figurative sense. With measurable values, 'sinken' is usually more natural; with quality or performance, 'abfallen' often sounds more precise.

Nach der Pause fällt seine Konzentration deutlich ab.
After the break, his concentration drops noticeably.
Im Vergleich zum letzten Monat sinkt die Nachfrage nach den Tickets.
Compared with last month, demand for the tickets is falling.