der Einbruch
break-in
The forcible and unlawful entry into a building, usually with the intention of stealing.
Die Polizei untersucht den Einbruch in das Juweliergeschäft.
The police are investigating the break-in at the jewelry store.

break-in / onset
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break-in
The forcible and unlawful entry into a building, usually with the intention of stealing.
Die Polizei untersucht den Einbruch in das Juweliergeschäft.
The police are investigating the break-in at the jewelry store.

'Der Einbruch' (the break-in) is the noun related to the separable verb 'einbrechen' (to break into something). This origin explains the literal meaning of forced entry.
Die Diebe sind nachts eingebrochen. Der Einbruch wurde erst am Morgen bemerkt.
The thieves broke in at night. The break-in was not noticed until the morning.
When referring to a theft, the location of the break-in is given with the preposition 'in' and the accusative.
Die Polizei untersucht den Einbruch in eine Bäckerei.
The police are investigating the break-in at a bakery.
In the meaning of 'Beginn' (beginning/onset), 'Einbruch' is used almost only in fixed expressions with times of day or seasons, usually with the preposition 'bei' (at/with) or 'mit' (with).
Die Wanderer müssen vor Einbruch der Dunkelheit zurück sein.
The hikers have to be back before the onset of darkness.
In an economic context, 'Einbruch' describes a sudden, sharp decline. Typical combinations are 'Einbruch der Aktienkurse' (drop in stock prices), 'Einbruch der Nachfrage' (drop in demand), or 'konjunktureller Einbruch' (economic downturn).
Die Reisebranche erlitt durch die Pandemie einen historischen Einbruch.
The travel industry suffered a historic slump because of the pandemic.
beginning
'der Beginn' and 'der Einbruch' can both mean a beginning, but they do not fit the same style. 'der Beginn' is neutral and general. 'der Einbruch' in this meaning often describes something suddenly setting in, especially with natural events or times of day, for example 'Einbruch der Dunkelheit' (nightfall / onset of darkness) or 'Einbruch des Winters' (onset of winter). Here, 'Einbruch' sounds stronger and more figurative. For normal appointments, courses, or events, 'Beginn' is more natural.

Mit dem Einbruch der Dunkelheit wurde es im Wald schnell kalt.
As darkness fell, it quickly became cold in the forest.
Der Beginn der Führung ist morgen um zehn Uhr.
The guided tour begins tomorrow at ten o'clock.