
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.

heart
The symbolic seat of a person's feelings, love, courage, and character.
Er ist ein Mensch mit einem guten Herzen.
He is a person with a good heart.

heart / center
The central, innermost, or most important part of something.
Wir spazierten durch das Herz der Altstadt.
We walked through the heart of the old town.

to take heart
An idiom meaning to gather all one's courage in order to do something difficult or unpleasant.
Er fasste sich ein Herz und sprach sie an.
He took heart and spoke to her.

to grow dear
An idiom meaning that over time you become very fond of a person or animal.
Obwohl der Hund anfangs scheu war, ist er uns schnell ans Herz gewachsen.
Although the dog was shy at first, he quickly grew dear to us.

to take to heart
An idiom meaning to take advice or criticism very seriously and think about it or adjust one's behavior accordingly.
Den Rat seiner Lehrerin hat er sich wirklich zu Herzen genommen.
He really took his teacher's advice to 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
Both words can refer to a person’s emotional and inner nature. Herz particularly emphasizes feelings such as love as well as moral character, while Seele refers more to the whole spiritual and emotional core of a person. Use Herz for a human emotional nuance and Seele for the more comprehensive innermost self.

Ihr gutes Herz zeigt sich daran, dass sie allen Nachbarn hilft.
Her good heart is evident in the way she helps all her neighbors.
Die Musik berührte seine Seele und ließ ihn lange nachdenklich bleiben.
The music touched his soul and left him thoughtful for a long time.