"Let her know."
I gave the answer "Deixa-a saber." The system response was: You used the command form "deixa-a" instead of the ele/ela form "deixe-a". The two correct translations given were "Deixe-a saber" and "Deixa ela saber." Was I really wrong to use the command with a?
yes, you can use "deixa-a", but that's rare, also cause of the connectionm"a-a" that make it one sound!,so, using "deixe-a" is better.
I'll give it a try: 1) her is a direct object rather than an indirect one hence lhe is not appropriate; 2) lhe should be used ahead of the verb of which it is the indirect object, or you needs the hyphen "verb-lhe" more European style.
The dictionary says that deixar takes direct pronoun after it: "deixar alguém louco" to drive someone crazy.
Would the "deixa" change to something else if it was "them", "us" or "me" following it? Also, why are both "deixa" and "deixe" allowed in this sentence?