"connaissons" is the conjugation for "nous" and it means "we are acquainted with".
"sais" is the conjugation for "je/tu" and it means "I/you know".
With people and places, "to know" translates to "connaître".
With things you have learned or before a verb in infinitive, "to know" or "to how to" translates to "savoir".
For anyone who (like me) misheard "sa fille" as "ça fille", and is wondering why it was marked as incorrect, see Sitesurf's explanation of the difference in the words here: https://forum.duolingo.com/comment/147320/Cela-ce-ca-cette-c-est
The same: "nous connaissons sa fille". French possessive adjectives, like all other adjectives, agree in gender and number with the noun they modify. "Fille" is feminine singular, so "sa" is feminine singular as well.
- Sa fille = his/her/its daughter
- Son fils = his/her/its son
- Ses enfants = his/her/its children