To compare (the most...)

To get this superlative form, just add an article in front of the comparative form of the adjective. 

  Ο ήλιος είναι ο πιο μεγάλος / ο μεγαλύτερος

   The sun is the biggest 

  Η Γη είναι η πιο μεγάλη / η μεγαλύτερη

   The Earth is the biggest 

  Το φεγγάρι είναι το πιο μεγάλο / το μεγαλύτερο

   The moon is the biggest 

It's nice to have a simple rule once in a while, don't you think.

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If any doubt about the comparative form of an adjective, check out the previous post about "more...than".