How do you know when to use nosotros and nosotras as to vosotros and vosotras?

The nosotros form is used when you are talking about a group of people, and you are including yourself in that group.

(example: we are not Italian. nosotros no somos Italianos)

The vosotros form is also speaking to a group of people but it is different for a couple of reasons:

1. You are speaking to a group of people that you are not including yourself in .

2. it is informal form (as opposed to Ustedes)- it is used in situations in which all of the people you are addressing are ones that you would individually us the tú form.

[example: Our house is red, your house is green

Nuestra casa es roja; la vuestra es verde]

Nosotros = we

Vosotros = all of you