heftig
violent / intense
Of great strength, intensity, or force. Often refers to natural events, pain, or physical reactions.
Ein heftiges Gewitter zog in der Nacht über die Stadt.
A violent thunderstorm passed over the city during the night.

violent / intense
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violent / intense
Of great strength, intensity, or force. Often refers to natural events, pain, or physical reactions.
Ein heftiges Gewitter zog in der Nacht über die Stadt.
A violent thunderstorm passed over the city during the night.

'Heftig' can describe a noun (attributively) or define the manner of an action as an adverb. As an adverb, it remains unchanged in the sentence.
Es gab einen heftigen Streit.
There was a heated argument.
In its basic meaning, 'heftig' describes strong physical forces. It is often combined with natural events or physical reactions, e.g. 'heftiger Regen' (heavy rain), 'heftiger Schmerz' (severe pain), or 'heftiger Aufprall' (violent impact).
Nach dem Unfall hatte er heftige Kopfschmerzen.
After the accident, he had severe headaches.
When it comes to human interactions, 'heftig' describes emotional intensity. Typical combinations are 'heftige Kritik' (harsh criticism), 'eine heftige Diskussion' (a heated discussion), or 'heftige Gefühle' (intense feelings).
Die Entscheidung des Chefs löste heftige Reaktionen aus.
The boss's decision triggered strong reactions.
In colloquial speech, especially among young people, 'heftig' is used as an exclamation of surprise or admiration. It then means something like 'krass' (crazy/intense) or 'beeindruckend' (impressive).
Du hast die Prüfung mit 1,0 bestanden? Heftig!
You passed the exam with a 1.0? Insane!
very
'sehr' and 'heftig' can both express that something is strong or intense. 'sehr' is a general intensifier and fits with many adjectives, adverbs, and some verbs. 'heftig' more often describes a strong effect, often with weather, pain, arguments, criticism, or strong feelings. Colloquially, 'heftig' can also show surprise. 'sehr' is more neutral, while 'heftig' sounds stronger and often more dramatic.

Nach dem Unfall hatte Tom heftige Schmerzen im Rücken.
After the accident, Tom had intense pain in his back.
Ich war sehr müde, weil die Reise so lange gedauert hat.
I was very tired because the trip took so long.