kräftig
strong
Refers to physical strength; muscular and robust.
Er ist ein kräftiger Mann und kann schwere Kisten heben.
He is a strong man and can lift heavy boxes.

strong / intense
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strong
Refers to physical strength; muscular and robust.
Er ist ein kräftiger Mann und kann schwere Kisten heben.
He is a strong man and can lift heavy boxes.

The adjective 'kräftig' (strong/intense) is derived directly from the noun 'die Kraft' (strength/power). Something that is 'kräftig' therefore has a lot of power, strength, or intensity.
Für diese Arbeit braucht man kräftige Arme.
You need strong arms for this work.
'Kräftig' can also be used as an adverb to intensify a verb. It then means something like 'strongly,' 'intensely,' or 'with a lot of energy.'
Vor Gebrauch die Flasche bitte kräftig schütteln.
Please shake the bottle vigorously before use.
In relation to the senses, 'kräftig' (intense) is often used for intense impressions. Typical combinations are 'ein kräftiger Kaffee' (a strong coffee), 'ein kräftiger Käse' (a strong cheese), 'eine kräftige Farbe' (a bold color), or 'ein kräftiger Geschmack' (an intense flavor).
Ich mag den kräftigen Geschmack von dunkler Schokolade.
I like the intense flavor of dark chocolate.
When it comes to food, 'kräftig' (hearty) describes a rich and filling meal. Common phrases are 'ein kräftiges Frühstück' (a hearty breakfast) or 'eine kräftige Suppe' (a hearty soup).
Ein kräftiges Frühstück gibt Energie für den ganzen Tag.
A hearty breakfast gives you energy for the whole day.
'Kräftig' (strong) also describes strong natural events or loud sounds. For example, people speak of 'ein kräftiger Wind' (a strong wind), 'ein kräftiger Regen' (heavy rain), or 'kräftiger Applaus' (strong applause).
Nach der Vorstellung gab es kräftigen Applaus für die Schauspieler.
After the performance, there was strong applause for the actors.
strong
'stark' and 'kräftig' are often close in meaning. Both can describe physical strength or clear intensity. 'stark' is very general and works for people, feelings, wind, coffee, or an effect. 'kräftig' often sounds more concrete: it describes a robust body, a full flavor, a rich color, or weather that is clearly noticeable. With people, 'kräftig' can also mean that someone is strongly built. For abstract things like 'ein starkes Argument' (a strong argument), 'kräftig' is usually not suitable.

Die Suppe schmeckt kräftig, weil viele Gewürze darin sind.
The soup has a strong flavor because it has a lot of spices in it.
Nach dem Training fühlt sich Ali wieder stark.
After training, Ali feels strong again.