
stärken
to strengthen
To make someone or something stronger, more resistant, or more influential.
Regelmäßiger Sport stärkt das Immunsystem.
Regular exercise strengthens the immune system.
to strengthen
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to strengthen
To make someone or something stronger, more resistant, or more influential.
Regelmäßiger Sport stärkt das Immunsystem.
Regular exercise strengthens the immune system.

to fortify oneself
To regain one's strength through eating and drinking.
Nach der langen Wanderung müssen wir uns erst einmal in der Hütte stärken.
After the long hike, we first need to fortify ourselves in the hut.

to starch
To treat laundry (e.g., shirt collars) with starch so that it becomes stiff and keeps its shape.
Früher war es üblich, die Kragen von Oberhemden zu stärken.
In the past, it was common to starch the collars of dress shirts.
The verb 'stärken' (to strengthen) is derived directly from the adjective 'stark' (strong). It means to 'make something or someone strong'.
Diese Erfahrung hat seinen Charakter gestärkt.
This experience strengthened his character.
'Stärken' (to strengthen) is often used with abstract concepts or body parts, for example 'das Selbstvertrauen stärken' (to strengthen self-confidence), 'die Wirtschaft stärken' (to strengthen the economy), 'die Muskeln stärken' (to strengthen the muscles), or 'jemandem den Rücken stärken' (to give someone support/back someone up).
Gezielte Übungen können die Rückenmuskulatur stärken.
Targeted exercises can strengthen the back muscles.
The reflexive form 'sich stärken' (to fortify oneself) means to regain strength through eating and drinking. It is often followed by the preposition 'mit' and the respective food in the dative.
Vor dem Wettkampf stärkte sich der Athlet mit Nudeln.
Before the competition, the athlete fortified himself with pasta.
The meaning 'Wäsche steif machen' (to make laundry stiff) is very specific and rather rare today. You find it in the context of housework, often referring to shirt collars or tablecloths.
Meine Großmutter hat ihre Tischdecken immer gestärkt.
My grandmother always starched her tablecloths.
to build up
After an illness, both aufbauen and stärken can have a positive meaning. Aufbauen emphasizes helping someone regain inner strength through comfort and encouragement, while stärken makes someone stronger or more resilient. Aufbauen is appropriate for emotional support; stärken is more suitable for increasing strength or resilience.

Nach der Krankheit stärkt das tägliche Training langsam seine Muskeln.
After his illness, daily training gradually strengthens his muscles.
Nach der Krankheit baut die Familie ihn mit viel Zuspruch wieder auf.
After his illness, his family builds him back up with lots of encouragement.