The English expressions "I have not seen him these three months," etc., should be translated "Hace or ha tres meses que no le veo," "Hace or ha tres meses que no le he visto," "No le he visto (or no le veo) desde hace tres meses," "Hace (or ha) tres meses desde que le ví la última vez."
"I have been writing these three hours," is translated "Hace tres horas que escribo (estoy escribiendo)," "Escribo" (estoy escribiendo) desde hace tres horas.
=Relative Pronouns=.
Cuyo in reality a relative possessive pronoun is often used as a pure relative instead of el cual followed by a noun, as—
Lo dí a un cliente, cuyo cliente lo pasó á un amigo de él:
I gave it to a customer which customer handed it to a friend of his.
The correct grammatical construction would be "el cual cliente"; but however much some grammarians disclaim this employment of cuyo, it is in the language and found in the best books and therefore must be accepted.
=Indefinite Pronouns=.
The Indefinite Pronouns algo and nada followed by an adjective, generally (but not necessarily) take =de= before the adjective, as—
Tengo algo (de) bueno: I have something good.
No tengo nada (de) bueno: I have nothing good.