umfallen
to fall over
To fall sideways or downward from a standing or upright position.
Durch den starken Wind ist der alte Baum im Garten umgefallen.
Because of the strong wind, the old tree in the garden fell over.

to fall over / faint / cave in
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to fall over
To fall sideways or downward from a standing or upright position.
Durch den starken Wind ist der alte Baum im Garten umgefallen.
Because of the strong wind, the old tree in the garden fell over.

'Umfallen' (to fall over/faint/cave in) is a separable verb. The prefix 'um-' goes to the end in a main clause. In subordinate clauses and in the infinitive, the verb stays together.
Pass auf, dass das Glas nicht umfällt!
Be careful that the glass doesn't fall over!
Since 'umfallen' (to fall over/faint/cave in) describes a change of state or position, the perfect tense is always formed with the auxiliary verb 'sein' (to be). So it is 'ich bin umgefallen' (I fell over/fainted/caved in), not 'ich habe umgefallen'.
Die Vase ist vom Tisch gefallen und umgefallen.
The vase fell off the table and fell over.
The literal meaning (m01) can refer to people and objects. The two colloquial meanings 'ohnmächtig werden' (to faint) (m02) and 'Meinung ändern' (to change one's opinion) (m03) always refer only to people.
Der Radfahrer ist gestürzt, aber zum Glück nicht umgefallen.
The cyclist crashed, but luckily didn't fall over.
When someone 'umfällt' (caves in) because they change their opinion, it often has a negative connotation. It implies that the person is weak, does not have a firm position, or gives in to pressure.
Er hat lange Widerstand geleistet, aber am Ende ist er doch umgefallen und hat zugestimmt.
He resisted for a long time, but in the end he did cave in and agreed.
to fall
'fallen' and 'umfallen' both have to do with downward movement. 'fallen' is more general: something or someone moves downward or falls to the ground. 'umfallen' is used especially when something was previously standing upright and then tips sideways or downward, for example a chair, a bottle, or a person. Colloquially, 'umfallen' can also mean that someone faints. Learners may confuse the words because in both cases something ends up down on the ground. The important point is: with 'umfallen', it is usually about losing an upright position.

Bei dem starken Wind ist die leere Mülltonne umgefallen.
The empty trash can fell over in the strong wind.
Der Schlüssel fällt vom Tisch auf den Boden.
The key falls from the table onto the floor.