'Used to' or 'Used for'?

‘Used to’ refers to something that was true in the past.

'I used to ride to work every day, but I live too far away now.'

‘Used to’ also means to be accustomed to.

'By the end of summer, I'm used to the heat.'

‘Used to’ refers to the purpose of something.

'Pens are used to write.'

You can also say ‘used for’ to mean the same thing, but you say ‘for writing’ and not ‘for write’.

'Pens are used for writing.'

Flickr CC: Daniel López