Learn Spanish (Latin America): El libro es difícil, pero lo comprendo. - The book is difficult, but I understand it.

In English, we say "I understand it," putting the object ("it") after the verb. In Spanish, direct object pronouns like lo, la, los, and las usually go directly before the conjugated verb. So, "I understand it" becomes lo comprendo (literally "it I understand").

Why do we use lo to mean "it" here instead of la?

The pronoun you use for "it" has to match the gender of the noun it replaces. Because it is referring back to el libro (the book), which is a masculine noun, we use the masculine pronoun lo. If we were talking about a feminine noun like la carta (the letter), we would use la and say la comprendo.

Why is the word for "I" missing before comprendo?

The verb comprendo comes from comprender (to understand). The -o ending already tells us that "I" (yo) am the one doing the action. You could say yo lo comprendo, but Spanish speakers usually drop the subject pronoun unless they want to add special emphasis.

Why do we use es instead of está for "is" in the first part of the sentence?