fromm
pious
Strongly religious and devout; following the rules and practices of a religion.
Sie war eine fromme Frau, die jeden Tag betete.
She was a pious woman who prayed every day.

pious / tame
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pious
Strongly religious and devout; following the rules and practices of a religion.
Sie war eine fromme Frau, die jeden Tag betete.
She was a pious woman who prayed every day.

You can recognize the meaning of 'fromm' (pious/tame) from context: If it refers to a person, it means 'religiös' (religious). In the fixed expression 'ein frommer Wunsch' (a pious wish), it means 'unrealistisch' (unrealistic). If it describes an animal, it means 'zahm' (tame).
Mein Großvater war ein sehr frommer Mann.
My grandfather was a very pious man.
In the religious meaning, 'fromm' (pious) is often combined with certain nouns and verbs. Typical collocations are 'ein frommes Leben führen' (to lead a pious life), 'ein frommer Christ sein' (to be a pious Christian), or 'ein frommes Gebet sprechen' (to say a pious prayer).
Sie versucht, ein frommes Leben nach den Regeln ihrer Religion zu führen.
She tries to lead a pious life according to the rules of her religion.
The meaning 'unrealistisch' (unrealistic) occurs almost exclusively in the fixed expression 'ein frommer Wunsch' (a pious wish). The form usually remains unchanged.
Seine Hoffnung, die Prüfung ohne Lernen zu bestehen, war nur ein frommer Wunsch.
His hope of passing the exam without studying was just wishful thinking.
The use of 'fromm' (tame) for 'zahm' (tame) with animals is somewhat old-fashioned and is used less often today. You often find it in the expression 'fromm wie ein Lamm' (as gentle as a lamb).
Das Lamm folgte dem Kind, so fromm war es.
The lamb followed the child, it was so tame.
holy / sacred
'heilig' and 'fromm' both belong to the religious sphere, but they do not describe the same thing. 'heilig' is usually said about places, things, times, or people that are considered especially venerated or divine in a religion. 'fromm' mainly describes people who are strongly religious and live according to their religion. Learners may confuse the words because both have to do with religion. As a practical guide: a place or object is heilig, while a religious person is fromm.

Meine Nachbarin ist sehr fromm und geht jeden Sonntag in die Kirche.
My neighbor is very pious and goes to church every Sunday.
Für viele Menschen ist dieser Berg ein heiliger Ort.
For many people, this mountain is a sacred place.