I posted this to Twitter earlier, so I thought I might as well share it here too. I've seen some folks trying to use this older form of the second person singular pronoun in stories, poetry, song lyrics and so on, but getting it all kinds of wrong, so I made a handy little guide.

thou - subject (see: 1st person 'I')
thee - object (see: 1st person 'me')
thy - dependent possessive (see: 1st person 'my')
thine - independent possessive (see: 1st person 'mine')

Examples from Shakespeare:

Sonnet 18:
Shall I compare THEE to a summer's day?
THOU art more lovely and more temperate
[...]But THY eternal summer shall not fade

Sonnet 134:
So, now I have confessed that he is THINE

'Thine' is also used in place of 'thy' if the following word begins with a vowel, for example:

Sonnet 132:
THINE eyes I love, and they, as pitying me

And, just so we're absolutely clear, 'thou' is ALWAYS singular! Plural of 'thou' is 'you'. 'You' has been adopted as a singular pronoun, but it didn't use to be. The plural form was used as a formal singular form, for politeness, respect or deference, but at some point people started to use it as the default 2nd person singular pronoun in informal as well as formal settings. There are other languages in which the 2nd person plural is used formally while the singular is informal, such as the difference between 'tu' and 'vous' in French.