I suppose your question is, why is it not "in mein Herz" or "in meinen Herz", etc.? The reason why "mein" turns into "meinem" and why "Herz" turns into "Herzen" is because it's dative. "In" is a preposition that can use accusative or dative; it uses accusative for movement and dative for location. When you're saying that something is in your heart, then that's a location, hence you need to use dative. The dative form of "mein" for "das-words" is "meinem", and "Herz" gets an extra "en" because it's treated like a weak noun (even though it's considered an exception), and so-called weak nouns get an extra "n" or "en" in accusative, dative, and genitive.
2 Answers By Expert Tutors
Claudia H. answered 07/09/19
Native German with 20+ Years of Teaching/Translating Experience
das Herz (the heart) - irregular declension:
Nominativ - das Herz - ein Herz
Akkusativ - das Herz - ein Herz
Dativ - dem Herzen - einem Herzen
Genitiv - des Herzens - eines Herzens
the preposition "in" is followed by the dative case, if no change in position occurs (in this case, if it is in your heart),
therefore it is: in meinem Herzen (=in my heart)
(PS: in colloquial language, especially related to the medical field, it is quite common to hear the strong declension "in meinem Herz" instead of the grammatically correct weak declension "in meinem Herzen")
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