to care / concern / languish
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to take care of
To look after someone or something, feel responsible, and do the necessary things for them or it.
Kannst du dich bitte um die Blumen kümmern, während ich im Urlaub bin?
Can you please take care of the flowers while I'm on vacation?
The verb 'kümmern' is related to the noun 'der Kummer' (worry, grief). 'Sich um etwas kümmern' (to take care of something) therefore means becoming active so that no trouble or worry arises. The meaning 'kümmerlich wachsen' (to grow poorly) describes a state of distress or weakness.
Aus Kummer über den Verlust aß er tagelang nichts.
Out of grief over the loss, he ate nothing for days.
The grammar reveals the meaning: 'sich kümmern um' is reflexive with a preposition and means 'to take care of'. The structure 'etwas kümmert jemanden' is not reflexive and means 'to worry someone' or 'to concern someone'. If the verb stands alone, it describes poor growth or failure to thrive.
Er kümmert sich um seine kranke Mutter.
He takes care of his sick mother.
In its most common meaning, 'kümmern' is a reflexive verb that always requires the preposition 'um' with the accusative. So you must always say: 'sich um etwas/jemanden kümmern' (to take care of something/someone).
Wer kümmert sich heute um das Abendessen?
Who is taking care of dinner today?
In this meaning, the verb is not reflexive. The person affected by something is in the accusative. This form is used almost exclusively in questions or negative sentences.
Was kümmert es mich, ob er kommt oder nicht?
What does it concern me whether he comes or not?
to interest
'interessieren' and 'kümmern' can both express that something is important to someone. The difference lies in the action. 'sich interessieren für' means: you find something interesting or want to know more about it. 'sich kümmern um' means: you take care of someone or something and do something for them or it. They can be confused because both have to do with attention. But interest alone is not yet responsibility. If you are only curious, 'interessieren' fits. If you help, organize, or provide care, 'kümmern' fits.
Am Nachmittag kümmert sich Mehmet um seine kleine Schwester.
In the afternoon, Mehmet takes care of his little sister.
Lisa interessiert sich für Medizin und liest viele Bücher darüber.
Lisa is interested in medicine and reads many books about it.