das Herz
heart
The central organ of the circulatory system that pumps blood through the body.
Regelmäßiger Sport ist gut für das Herz.
Regular exercise is good for the heart.

heart
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heart
The central organ of the circulatory system that pumps blood through the body.
Regelmäßiger Sport ist gut für das Herz.
Regular exercise is good for the heart.

'Herz' (heart) belongs to a small group of neuter nouns with a special declension. In the dative and genitive, it takes the ending '-en': 'dem Herzen' (to the heart) and 'des Herzens' (of the heart). In the nominative and accusative, it remains 'das Herz' (the heart).
Er folgte der Stimme seines Herzens.
He followed the voice of his heart.
Many words that have to do with feelings are derived from 'Herz' (heart). These include the adjective 'herzlich' (warm/friendly), the noun 'das Herzklopfen' (a pounding heart, from excitement), and the phrase 'herzlichen Glückwunsch' (heartfelt congratulations).
Sie wurde von den Gastgebern sehr herzlich empfangen.
She was welcomed very warmly by the hosts.
In its symbolic meaning, 'Herz' (heart) is often combined with adjectives that describe character. Typical expressions are 'ein großes Herz haben' (to have a big heart / be generous), 'ein kaltes Herz haben' (to have a cold heart / be unfeeling), or 'ein gebrochenes Herz haben' (to have a broken heart).
Er hat ein großes Herz für Tiere und hilft, wo er kann.
He has a big heart for animals and helps wherever he can.
To describe the central part of something, 'Herz' (heart) is often used with the genitive or the preposition 'von' (of). For example, people speak of the 'Herzen der Stadt' (heart of the city) or the 'Herzen des Problems' (heart of the problem).
Der alte Marktplatz bildet das Herz der Stadt.
The old market square forms the heart of the city.
The idiom 'sich ein Herz fassen' (to take heart) is reflexive and uses the dative. The structure is always: subject + 'fasst sich' (dative) + 'ein Herz'.
Endlich fasste sie sich ein Herz und sagte ihm ihre Meinung.
Finally, she took heart and told him her opinion.
In the expression 'jemandem ans Herz wachsen' (to grow dear to someone), the person who becomes fond of someone or something is in the dative. The subject of the sentence is the person or thing that one becomes fond of.
Die Stadt ist mir in den letzten Jahren sehr ans Herz gewachsen.
The city has grown very dear to me over the last few years.
The expression 'sich etwas zu Herzen nehmen' (to take something to heart) is reflexive with the dative. The person is the subject, and what one takes seriously is the accusative object.
Du solltest dir den Rat deiner Mutter wirklich zu Herzen nehmen.
You should really take your mother's advice to heart.
soul
'Herz' and 'Seele' can both talk about a person's inner life, that is, about feelings, love, and personality. 'Herz' is often used when talking about feelings, kindness, courage, or love. 'Seele' refers more to a person's whole inner being: what they feel, think, and carry deep inside. The words can be confused because both can mean more than something physical. As a simple guide: 'Herz' often sounds closer to feeling and love, while 'Seele' is closer to a person's deep inner self.

Sie hat ein gutes Herz und hilft den Nachbarn oft.
She has a good heart and often helps the neighbors.
Nach dem langen Gespräch fühlte sich seine Seele leichter.
After the long conversation, his soul felt lighter.