Learn Italian: Lui esprime liberamente quello che ha in mente. - He freely expresses what he has in mind.

When "what" means "that which" or "the thing that" in a statement, you can't use the question word cosa. You need a compound relative pronoun like quello che (literally "that which"). The word che on its own just means "that" or "which" and needs to connect to an existing noun, while quello che acts as both the noun and the connector.

Could I use ciò che instead of quello che in this sentence?

Yes! Ciò che means exactly the same thing. Quello che is very common in everyday spoken Italian, while ciò che can sound slightly more formal or elegant. Both are perfectly correct here: Lui esprime liberamente ciò che ha in mente.

How does the adverb liberamente (freely) get its ending?

In Italian, you create many adverbs by adding -mente to the feminine singular form of an adjective, much like adding "-ly" in English. The adjective for "free" is libero. Its feminine form is libera, so adding -mente gives you liberamente.

Why is it just in mente and not nella mente (in the mind)?