I'm not actually a native speaker (someone please correct me if I'm wrong), but that would be
"elle sait qu'elle doive parle en francais autour de lui".
This is a difficult sentence since it requires the present subjunctive of devoir (an irregular verb), a mood which is not often used in English. There will be lessons on this at the end of the French tree.
She can speak french would be: Elle peut parler francais...
She knows to speak french does make sense... someone telling someone else that she knows to speak french in whatever environment they are in...
Where does the "how" show up in this sentence?
I wrote: she knows to speak french, which translates literally and makes sense to me, but it was marked incorrect....
This makes sense to me... thank you...
I'm wondering... if "she" was in France at someone's house who did not speak English and say, her husbands mother asked him, "Does she know to speak French here?" and he answered, "Yes, she knows to speak French."... would that be written differently?
Because it simply isn't. This is the way the sentence "she knows how to speak French" is written in French.
You version, <<Elle sait comment parler Francais>> seems wrong to me, it feels unnatural, but not being a native speaker, I cannot say for certain that it would not be acceptable.