unterschreiten
to fall below
To drop below a certain limit, measurement, or fixed value, or to fail to reach it.
Die Temperatur hat in der Nacht den Gefrierpunkt unterschritten.
The temperature fell below freezing during the night.

to fall below / fail to meet
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to fall below
To drop below a certain limit, measurement, or fixed value, or to fail to reach it.
Die Temperatur hat in der Nacht den Gefrierpunkt unterschritten.
The temperature fell below freezing during the night.

'Unterschreiten' (to fall below / fail to meet) is an inseparable verb. The prefix 'unter-' remains firmly attached to the verb in all tenses and sentence constructions.
Er hat die vorgeschriebene Zeit leider unterschritten.
Unfortunately, he came in under the required time.
The word is made up of 'unter' (under) and 'schreiten' (to step, to walk). Figuratively, it describes going below a line or level.
Die Verkaufszahlen haben die Erwartungen deutlich unterschritten.
The sales figures fell clearly below expectations.
The verb 'unterschreiten' (to fall below / fail to meet) is always used with an accusative object. You ask: whom or what is being fallen below?
Der Läufer unterschritt den alten Rekord um drei Sekunden.
The runner beat the old record by three seconds.
When the context is about numbers, measurements, or concrete limits (e.g., temperature, price, time), the first meaning is intended. When it is about abstract things like quality or expectations, the second meaning is intended.
Der Aktienkurs hat die 100-Euro-Marke unterschritten.
The share price fell below the 100-euro mark.
to reach
'erreichen' and 'unterschreiten' are often opposites when talking about goals, limits, or values. 'erreichen' then means that you get to the desired value, goal, or standard. 'unterschreiten' means that you stay below it or drop below a limit. The words can be confused because both occur with numbers, performance, or requirements. The important difference is: if you 'erreichen' a goal, you meet it. If you 'unterschreiten' a minimum value, you specifically do not meet it.

Bei der Prüfung darf die Punktzahl den Mindestwert nicht unterschreiten.
In the exam, the score must not fall below the minimum value.
Im Zug erreiche ich mit dem Handy endlich wieder Empfang.
On the train, I finally get phone reception again.