Learn Polish: Łatwo to zrozumieć. - It is easy to understand this.

In English, we say "It is easy to do something" using an adjective. But Polish uses an impersonal construction with an adverb plus an infinitive. So you must use the adverb łatwo when describing an action like the infinitive zrozumieć.

How do we know to use the -o ending for this adverb instead of -e?

Most adjectives ending in a hard consonant + -y, like łatwy, form their adverbs by dropping the -y and adding -o. The -e ending is mostly used for adjectives that end in soft or hardened consonants (like dobry becoming dobrze).

Why is to placed before the verb instead of after it?

While Łatwo zrozumieć to is grammatically correct, Polish pronouns usually avoid the very end of a sentence unless you want to strongly emphasize them (as in, "It's easy to understand THIS"). Putting to between the adverb and the verb sounds much more natural.

I've learned rozumieć for "to understand". Why is it zrozumieć here?

Zrozumieć is the perfective version of rozumieć. Perfective verbs focus on the completed result of an action. Since the sentence talks about successfully grasping a specific point ("understanding this"), rather than the ongoing process of understanding, the perfective form is used.