How could I know when to add those words?
For instances, Shut up
Phrasal verbs are framed with the combination of (verb + preposition) or (verb + adverb)
But phrasal verbs have completely different meanings from the usual one.
don't split (shut + up). It is one verb i.e. phrasal verb (shut up).
Similarly : write down, note down, clean up, etc.
Here is some list of phrasal verbs and their meanings :