The subject pronoun ellas means they in English. Since in this sentence you are talking about more than one person, that is, you are talking about two people María y Ana, you need to use the subject pronoun that matches the third person plural in feminine form for María and Ana are women, hence we use ellas. On the other hand, ella means she, vosotras means you (feminine plural), and nosotras means we (feminine plural).
The subject pronoun nosotros means we in English. Given that this sentence talks about Ramón y yo (Ramón and I), then this sentence stands for more than one person. Hence we must use the subject pronoun that matches the first person plural, which is we. On the other hand, yo means I, vosotros means you (masculine plural), and ella means she (feminine singular).
The subject pronoun yo means I in English. Given that I need to talk about myself, then I need to use the subject pronoun that matches the first person singular. I can't use vosotros because this stands for the second person plural, usted is the formal you (singular form) and ustedes is similar to vosotros but it's used in Spain in formal way. However, ustedes is used in Latin America either in formal or informal way.
If I want to talk to my friends I'd use the subject pronoun ustedes. As we said above, this is used in Latin America either in formal or informal way. We can't use usted here because this is used when talking to a person in formal way, ellos is used when talking about more than one person and tú is used to talk to a person in informal way.
The reason why we choose usted here is because my teacher is a person to whom I must show respect. Hence usted allows me to do this because it's the polite version of you (singular form). On the other hand, ellas means they, vosotras means you (plural and informal in Spain) and ella means she.
They is used in English to talk about more than one person either male or female form alike. In Spanish, there are two forms of they. First, ellas that you use to talk about only female and second ellos that you use when talking about male (or even male and female together). So here we choose ellos.
We is used in English to talk about more than one person either male or female form alike. In Spanish, there are two forms of we. First, nosotros that we use when talking about us and is used for male form (or even male and female together) and second nosotras that we use to talk about us but only in female form. Since we must talk about all females, we use nosotras.
Él is used to talk about an only person and this person is given in male form. Therefore, this subject pronoun stands for the third person singular and means he in English. Recall that she means ella, usted is the formal you (singular form) and yo matches the first person singular translated as I.
Ella is used to talk about an only person and this person is given in female form. Hence this subject pronoun stands for the third person singular and means she in English. On the other hand, nosotras means we (feminine plural form), vosotras is the informal you (feminine plural form) and él means he.
Yo is used to talk about an only person and this person can be male or female. Hence this subject pronoun stands for the first person singular and means I in English. On the other hand, nosotros means we (masculine plural form), ella means she and usted is the formal you.
