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

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 support
'unterstützen' and 'stärken' can both mean that someone or something ends up in a better position. Use 'unterstützen' when you help, for example with money, time, work, or approval. 'stärken' means more specifically to make someone or something stronger, more confident, or more influential. The words can be confused because help often also makes someone or something stronger. Still, the focus is different: 'unterstützen' describes the help itself, while 'stärken' describes the result.

Im Büro stärkt ein gutes Gespräch oft das Vertrauen im Team.
At the office, a good conversation often strengthens trust within the team.
Wir haben unseren Nachbarn beim Umzug unterstützt.
We supported our neighbors with their move.