Learn Spanish (Latin America): Lo eterno perdurará mientras exista la verdad. - What is eternal will endure as long as the truth exists.

Because the sentence is talking about a future, indefinite condition. When mientras means "as long as" and refers to an anticipated future timeframe (set up here by the future tense verb perdurará), it requires the present subjunctive. If you used the indicative mientras existe, it would describe a habitual or current fact, meaning "while the truth exists (right now)."

I know el and la, but what exactly is lo doing in Lo eterno?

Lo is the neuter article. When you combine lo with a masculine singular adjective like eterno, it turns the adjective into an abstract noun. It translates to "that which is eternal," "the eternal thing," or "what is eternal." This is a very common way to express abstract concepts in Spanish.

Could I just use durará instead of perdurará here?

You could use durará (will last), but perdurará (will endure or live on) is a stronger, more evocative choice. Adding the prefix per- to durar gives the sense of something lasting a very long time or surviving through the ages, which perfectly fits the philosophical tone of Lo eterno.

Why is the subject la verdad placed after the verb ?