"The store is in the new apartment building."
Translation:Le magasin est dans le nouvel immeuble.
Remember the BANGS rule, an earlier lesson. So easy to forget these things - I do it all the time too. https://forum.duolingo.com/comment/25017515/BANGS-rule
el is replacing eau in adjectives when the following word begins with a vowel sound, for euphonie reasons. It rolls better of the tongue.
To help you with that, try to put the adjective in the feminine form : Nouveau (masculine) and Nouvelle feminine. Every adjective following that rule should take the ending el when in front of a masculine noun beginning with a vowel sound.
N.B : I guess nouveau and beau are the only instances. Another one would be jumeau but jumel wouldn't make sense because jumeau always comes after the noun it describes (-> un frère jumeau).
N.B bis : In the same idea, bon is pronounced like its feminine "bonne" whenever it is placed in front of a vowel sound (only pronounced like it though, still spelled the same). That is why writing bonne anniversaire is a common mistake French speakers tend to make.
"dedans" means inside it. Saying dedans le would sound like saying inside it it.
An example of how to use dedans : "Il y a un immeuble, et le magasin est dedans."
Please note that you could find occurrences of "dedans le/la" in outdated texts or as a stylistic device in some songs or poems.