This is the 339th most frequent German word.
alten
"Alten" is the plural form of "alt" in German, meaning "old" when describing people or things.
In this sentence, 'alte' is the accusative singular form of 'alt', describing the neuter noun 'Museum'.
Ich besuchte das alte Museum in der Stadt.
I visited the old museum in the city.
Here, 'alte' is the nominative singular masculine form of 'alt', modifying the noun 'Mann'.
Der alte Mann ging über die Straße.
The old man crossed the street.
In this case, 'alten' is the dative singular feminine form of 'alt', referring to the noun 'Bibliothek'.