How do I know which subject pronoun to use for the second person (you)?

Translating "you" into Spanish can be tricky because where we only have one option in English, there are several different "you" forms in Spanish.

Which one you choose to use depends on how well you know the person you are talking to or writing to, whether you are talking to one person or more and sometimes whether the person or people are male or female.

Tú

Tú is used when you are talking to one person, male or female, that you know well: for example a friend or relative. It is therefore known as a familiar form.

Vosotros/Vosotras

Vosotros and vosotras are also familiar forms, but are used when you are addressing more than one person. For male groups or groups comprising both men and women, vosotros is used, whereas for groups of just females, vosotras is used.

Note: Vosotros and vosotras are used exclusively in Castillian Spanish (ie. in Spain) and not in Latin America (Ustedes is used instead: see below).

Usted

Usted is used when you are addressing one person, but someone that you do not know very well or who has a higher status or is older than you. It is therefore known as the formal or polite form.

Ustedes

Ustedes is another formal form, but it is used to address more than one person, either male or female.

Note: The familiar forms have become more and more customary in recent years and are now the norm in many situations which were previously regarded as formal.