perturbing...................

re. Sabes, creo que esto es perfecto. (and this sentence is perfect too)

if i had written this without google translate, it would have been:

sabes gue penso (pienso) este (que esto) es perfecto

the sentence that you would have written without google translate makes perfect sense in spanish and is perfectly understandable to any spanish speaker so you should be proud of yourself..................

and for this:
ya ha mentido antes de (ya ha mentido antes)

antes and después are followed by the preposition de but only when they are followed by a noun phrase, by an infinitive verb phrase or by the equivalent in spanish to a that-clause............when used isolated they work as an adverb so they take no preposition after them...........

could i have written:
Él ha mentido anteriormente. (just perfect............normally we would add 'ya' and say 'él ya ha mentido anteriormente'.............ya is already in english)