These words are very similar to each other. Considering the structure or the surface of an object, you can use eather hart or fest. example: Das Holz ist hart. = Das Holz ist fest. Sometimes, it's a matter of feeling which one sounds better. Native speakers won't blame you for choosing the ,,wrong" word in a special context. However, ,,fest" can also be used as a synonym for ,,stark" (strong): - jemand glaubt fest an etwas. (somebody strongly believes in sth.) - jemand hat einen festen Willen - jemand leistet festen Widerstand ,,hart" can be used as a synonym for ,,schwierig" (tough): - harte Zeiten - eine harte Aufgabe etc. I hope that helped a bit!

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