Can you explain 'lan'. Sometimes I see it and sometimes I don't. It seems to replace 'nan' in all its meanings. Is this true?

It's one of the Haitian Creole singular definite articles.  It usually comes after words that have  nasal sound and end with a consonant.  Some people do use "lan" instead of "nan".
Some people say plim lan, madanm lan, machin lan instead of plim nan, madanm nan, machin nan.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words