Grace tiene una guitarra.
Yo tengo means "I have"
Nosotros tenemos means "We have"
In this sentence, we are talking about Grace, which is Ella/El tiene "She/He has"
Hope this helps! :)
Los amigos son simpáticos.
Los because you can see amigos has an O and not an A ; also simpáticos has an O and not an A which indicates it is masculine and Los is masculine, Las is not
B. mis, mi. The first one is plural and second is singular.