"¡Tienes que acordarte de eso!"

Recordar is transitive and always needs a direct object - "Yo recuerdo algo." Acordarse is intransitive and doesn't need a direct object - "Me acuerdo." If you want to add a direct object to Acordarse, you must use "de" - "te acuerdas de algo?"

Better use a dictionary. It's quite long to explain. The dictionary has good examples.

Some verbs like acordar must be followed by "de" when there is a direct object as Juicy_Maffews explained above. Other verbs that also follow that rule are listed here: https://www.thoughtco.com/verbs-followed-by-de-and-an-infinitive-3079236