"You know my dog."
Translation:Sie kennen meinen Hund.
"Sie kennen meinen Hund." I think in this example, the person is talking to a stranger or probably his or her boss so "Sie" was used. Remember that Sie could mean "she", "them" or "you" formal. If you're confused, just look at the verb used here (kennen). Kennen can't be used for "du" coz the verb will be conjugated to "kennst". It can't also be used for 3rd person singular/sie coz the verb will be kennt. Hope it helps
You cannot use kennst if you are using "sie".
Verb: Kennen If you conjugate this verb, it'll be like this:
Singular: Ich kenne (I) Du kennst (You informal) Er/sie/es kennt (he/she/it)
Plural: Wir kennen (We) Ihr kennt (You all) sie/Sie kennen (they/You formal)
Hope it helps
This is also correct if you are talking to someone you know like your friends or family members. But to be more polite, "Sie" was used in the example. Duo said "Sie kennen meinen Hund." This is quite tricky coz it could also mean "she knows my dog" or "they know my dog". If you're having trouble deciding which person was used, just look at the verb. It'll help you decide which pronoun suits the sentence. In this example, "kennen" was used so you have to choose "Sie" formal.