It's sometimes not a bad idea to make sure some of that "old knowledge" is still accurate, so let's get back to basics: adjective declensions with definite articles. To make it as easy as possible, remember the following three rules for adjective endings:
1. All adjectives in the singular nominative case end in -e, regardless of noun gender. Here is an example with the singular masculine nominative:
Der alte Minister ging in den Saal.
The old minister went into the hall.
Caption 37, Märchen - Sagenhaft - Des Kaisers neue KleiderPlay Caption
2. All adjectives in the dative or genitive cases, as well as all adjectives in plural form, end in -en regardless of the noun's gender. Here is an example with the plural genitive:
Der Kaufmann ließ es sich schmecken und verbrachte die Nacht in einem der schönen Schlafräume.
The Merchant enjoyed the meal and spent the night in one of the beautiful bedrooms.
Captions 25-26, Märchen - Sagenhaft - Die Schöne und das BiestPlay Caption
3. This is the hard one: Adjectives for masculine nouns in the singular accusative case end in -en, but adjectives for feminine and neuter nouns in the singular accusative case end in -e. Here is an example with the single masculine accusative:
So gibt es hier die elektrische Gitarre für den abgebrühten [Umgangssprache] Rockstar.
Thus there is the electric guitar here for the jaded rock star.Play Caption
If the noun above had been feminine (or neuter), it would have dropped the -n: ...für die abgebrühte Musikerin.
There is a system for remembering adjective endings that many people find easy to remember called the Oklahoma — see if it works for you. (A special hat tip to Yabla subscriber Mike S. for that!) You can also search for some of your favorite adjectives on Yabla German and review the ways the adjectives end in the different cases with different noun genders.