häufig
frequent
Recurring at short intervals or often; happening many times.
Er stellt während des Unterrichts häufig Fragen.
He frequently asks questions during class.

frequent
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frequent
Recurring at short intervals or often; happening many times.
Er stellt während des Unterrichts häufig Fragen.
He frequently asks questions during class.

The word 'häufig' (frequent/often) is related to 'der Haufen' (a large amount/pile). It describes that something occurs in large numbers or often, in other words 'haufenweise' (in heaps/lots).
Technische Probleme sind ein häufiger Grund für Verspätungen.
Technical problems are a frequent reason for delays.
As an adverb, 'häufig' (frequently/often) usually stands in the middle of the sentence, often after the conjugated verb and the subject. It describes how often an action takes place.
Wir gehen häufig im Park spazieren.
We often go for walks in the park.
The comparative forms 'häufiger' (more often) and 'am häufigsten' (most often) are often used as adverbs to compare frequencies. 'Am häufigsten' often stands in the middle of the sentence or at the beginning of the sentence for emphasis.
Seit er näher an der Arbeit wohnt, kommt er häufiger mit dem Fahrrad.
Since he has lived closer to work, he comes by bike more often.
much / a lot
'viel' and 'häufig' can both describe that something happens often. But 'viel' is broader: it can also mean a large amount or strong intensity. 'häufig' is more precise when an action happens again and again or occurs at short intervals. Learners confuse the words because sometimes you can say both, for example 'Ich fahre viel Zug' and 'Ich fahre häufig Zug' (I travel by train a lot / frequently). 'häufig' sounds more precise for repetition, while 'viel' sounds more general and more everyday.

Daniel sieht im Garten häufig kleine Vögel.
Daniel often sees small birds in the garden.
Der Fahrer ist diese Woche viel unterwegs.
The driver is on the road a lot this week.