Phrasal verbs are mainly used in spoken English and informal texts. They consist of a verb plus a particle (preposition, adverb). The particle can change the meaning of the verb completely, e.g.:
look for = seek (look for her ring)
look up = consult a reference book (look a word up in a dictionary)
look forward = anticipate with pleasure (look forward to meeting someone)
The particle is placed either after the verb or after the object.
Write down the word. / Write the word down.
If the object is a pronoun, however, the particle has to be placed after the pronoun (object).
Write it down.
There are no rules that might explain how phrasal verbs are formed correctly - all you can do is look them up in a good dictionary and study their meanings.
Here you can find a list of the most common phrasal verbs, with meanings and examples.