How to distinguish whether it is written כ or ח in a word? For example: לחם and לכם? Do these letters pronounce differently?
Most Hebrew speakers pronounce these letters the same way - /x/, and you'll just have to learn how to distinguish between them, based on context and experience. Some Hebrew speakers, mainly Sephardi and Mizrahi, do pronounce these two letters differently - כ is /x/, and ח is /ħ/. You can read more here - https://en.wikipedia.org/wiki/Modern_Hebrew_phonology
So, "לכן" is to a group of girls (or at least to more than one girl), and "לך" is to one person...for a guy it would be "l'cha" and for a girl it would be "lach". Also, just to throw it in with the others, "לכם" would be to a group of people. "לכן" makes it specifically to girls. Hope that helps!
אתן is the personal pronoum for "you" (for femenine plural), and לכן means "to you" (also femenine plural)
I put "you have a dog" (which is what it says above), but it was marked wrong as it says the correct answer is "You girls have a dog". Why girls??
How come sometimes יש comes before the pronoun, but other times after? I can't figure out the pattern