"Make a nice dinner for mom!"
Translation:Fate una bella cena per la mamma!
So, paradoxically, even though you often omit the article when talking about close relatives, in an informal context where you're talking about someone with affection (e.g. using mamma not madre) you often use the article, e.g. when talking to other friends about my friend Valentina, I and all her other friends would typically refer to her as la Vale.
Technically the imperative would be fai, which is then truncated to fa'; the same happens for dici (di'), vai (va'), dai (da') and stai (sta'). However the rules for accents and apostrophes on monosyllables haven't been constant throughout the history of the language, and not all grammarians agree with the rules taught in school; the Treccani for instance allows fa as long as the next consonant is pronounced as doubled (phono-syntactic doubling) and the Crusca (somewhat timidly) lists dì. Duolingo isn't consistent on this.
Bello in some cases changes it's meaning when it's before or after some specific nouns:
Bella cena = nice, rich dinner Cena bella = beautiful as in good looking but that doesn't make sense with dinner
Also take note of this:
Vecchio amico = old (long time) friend Amico vecchio = old (aged) friend