Learn Spanish (Spain): ¿Sabes la verdad?

No, sabes is completely regular! For the verb saber (to know), only the 'I' form is irregular: yo sé (I know). The 'you' form follows the normal rules for an -er verb, which gives us tú sabes (you know).

Could I use conoces here instead of sabes? They both mean 'to know', right?

You have to use sabes here. In Spanish, saber is used for knowing facts, information, or how to do something (like knowing the truth). Conocer is used for being familiar with people, places, or things (like knowing a person or a city).

Why isn't there a word for 'you' in the Spanish sentence?

In Spanish, the verb ending usually tells you who is doing the action. Because sabes can only mean 'you know' (informal singular), adding the word tú (you) isn't necessary. You would only add it if you really wanted to emphasize it.

How do we know truth is feminine (la verdad) since it doesn't end in -a?

While most feminine words end in -a, words that end in -dad or -tad are almost always feminine in Spanish. That is why it is la verdad (the truth), just like la ciudad (the city) or la universidad (the university).