to rear-end / merge
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to rear-end
In road traffic, to crash with one's own vehicle into a vehicle that is driving ahead or is stopped.
Wegen des plötzlichen Staus sind mehrere Autos aufeinander aufgefahren.
Because of the sudden traffic jam, several cars rear-ended one another.
to merge onto
To drive from a smaller road onto a larger road, especially a highway or expressway.
An der nächsten Anschlussstelle müssen wir auf die Autobahn auffahren.
At the next interchange, we have to merge onto the highway.
to put on
To serve and present a large and usually impressive amount of food and drink.
Zum Jubiläum hat das Hotel ein riesiges Buffet aufgefahren.
For the anniversary, the hotel put on a huge buffet.
to flare up
To suddenly and violently show an angry, startled, or aggressive reaction.
Als sie ihn kritisierte, ist er sofort aufgefahren und hat den Raum verlassen.
When she criticized him, he immediately flared up and left the room.
to deploy
To bring heavy technical equipment or military equipment, especially guns, into a particular position.
Die Artillerie fuhr ihre Geschütze auf dem Hügel auf.
The artillery deployed its guns on the hill.
'Auffahren' (to rear-end / merge) is a separable verb. The prefix 'auf' comes at the end in main clauses. In subordinate clauses or with the infinitive with 'zu', the verb stays together.
Er fährt an der nächsten Ausfahrt auf die Autobahn auf.
He merges onto the highway at the next exit.
The choice of auxiliary verb in the perfect tense depends on the meaning. For all meanings that describe movement or a sudden change of state, such as an accident, entering the highway, or becoming angry, use 'sein' (to be). Only when it is about serving food do you use 'haben' (to have).
Der LKW ist auf ein Stauende aufgefahren.
The truck rear-ended the end of a traffic jam.
In the meaning 'to cause an accident', the preposition 'auf' with the accusative is often used to name the obstacle. The subject is the vehicle coming from behind.
Mein Auto ist auf das vordere Fahrzeug aufgefahren.
My car rear-ended the vehicle in front.
When 'auffahren' (to put on) is used in the context of food, it means to serve a large, impressive amount of food. It is often used without a specific object or with general terms such as 'ein Buffet' (a buffet).
Bei der Hochzeit wurde ein riesiges Buffet aufgefahren.
A huge buffet was put on at the wedding.
This figurative meaning describes a sudden, violent, and usually negative emotional reaction. The verb is used here without an object and has no literal connection to driving.
Als ich ihn auf den Fehler hinwies, ist er sofort aufgefahren.
When I pointed out the mistake to him, he immediately flared up.
to rebound / bulge
Prallen generally describes a forceful impact against a surface or obstacle. Auffahren specifically refers to a traffic accident in which one vehicle hits a vehicle ahead of it from behind, whether that vehicle is moving or stopped. For this particular type of accident, auffahren is more precise.
Der Wagen fuhr von hinten auf das wartende Auto auf.
The car rear-ended the waiting vehicle.
Der Wagen prallte seitlich gegen das andere Auto.
The car hit the other car on the side.
