Learn Italian: Non puoi giustificare un bugiardo.

Just like in English, Italians often use the informal "you" (tu) to make general statements. While the impersonal non si può (one cannot) is grammatically perfect and slightly more formal, saying non puoi is very natural in everyday conversation to mean "people in general cannot".

How does the word bugiardo (liar) connect to other Italian words?

It comes directly from bugia, which means "lie". The suffix -ardo is sometimes used to describe a person who frequently does a specific (usually negative) action, making un bugiardo someone who tells lies. For a female liar, you would say una bugiarda.

Can giustificare mean anything else besides "to justify"?

Yes! While it perfectly translates to "justify" in this moral sense, it is also the standard word used in Italian schools for "to excuse" an absence. A parent's written note to excuse their child is called a giustificazione.