Learn Spanish (Spain): Entiendo el catalán, pero no lo hablo. - I understand Catalan, but I don't speak it.

In Spanish, you usually need a definite article (el) before the name of a language. You generally only drop the article when the language name comes directly after verbs like hablar (to speak) or aprender (to learn), or prepositions like en or de. Since the verb here is entender (to understand), the el is required.

Why is the pronoun lo placed before the verb in no lo hablo?

Direct object pronouns like lo (which means "it" here, replacing the language) must go immediately before a conjugated verb in Spanish. Unlike English, where we say "I do not speak it", Spanish structures this as "I not it speak".

If we don't use el after the verb hablar, why do we use lo in no lo hablo?

It is true that if you named the language, you would just say no hablo catalán without the el. However, when you replace the noun with a pronoun meaning "it", you must use the standard direct object pronoun lo. The rule for dropping the article only applies to the actual name of the language.

Why does entiendo have an ie when the infinitive is entender?