begrenzen
to border / bound
To define or form a spatial or physical boundary for something.
Eine hohe Mauer begrenzt das Grundstück zur Straße hin.
A high wall borders the property on the street side.

to limit / border
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to border / bound
To define or form a spatial or physical boundary for something.
Eine hohe Mauer begrenzt das Grundstück zur Straße hin.
A high wall borders the property on the street side.

'Begrenzen' (to limit/border) is derived from the noun 'die Grenze' (border/boundary). The prefix 'be-' makes it an inseparable verb that describes an action: setting a boundary.
Der Fluss bildet die natürliche Grenze zwischen den beiden Ländern.
The river forms the natural border between the two countries.
In its abstract meaning, 'begrenzen' (to limit) is often used for non-physical things. Typical combinations are: limit costs, time, speed, choices, or the number of participants.
Die Firma muss ihre Ausgaben für das nächste Quartal begrenzen.
The company has to limit its expenses for the next quarter.
The reflexive form 'sich begrenzen auf' (to limit oneself to) has a fixed structure. It always uses the preposition 'auf' followed by the accusative and means that you deliberately focus on or restrict yourself to something.
Ich begrenze mich in meiner Präsentation auf die wichtigsten Fakten.
In my presentation, I am limiting myself to the most important facts.
to reduce
'reduzieren' and 'begrenzen' are both connected with having less or not too much of something. 'reduzieren' means that an amount, a value, or an intensity becomes smaller or is made smaller. 'begrenzen' means that you set a limit, for example for time, costs, or the number of participants. Something can be limited without having been reduced beforehand. Learners may confuse the words because both can lead to less. Memory sentence: 'reduzieren' makes something smaller; 'begrenzen' sets a maximum.

Der Verein begrenzt die Zahl der Teilnehmer auf zwanzig Personen.
The club is limiting the number of participants to twenty people.
Die Firma reduziert die Kosten, weil die Einnahmen gesunken sind.
The company is reducing costs because revenue has fallen.