Learn Portuguese (Portugal): É incrível quão rápido eles conseguem reter a imagem mental. - It is incredible how fast they can retain the mental image.

In English, "how" is used for both methods ("how does it work?") and degrees ("how fast is it?"). In Portuguese, como is only used for methods. When you want to express the degree or intensity of an adjective or adverb (like "how fast" or "how big"), you must use quão or o quão.

Could I say o quão rápido in this sentence instead of just quão rápido?

Yes! In indirect degree clauses like this one, quão and o quão are completely interchangeable. Adding the article o is very common in everyday spoken Portuguese, while dropping it can sound slightly more formal or literary, but both mean exactly the same thing here.

I thought rápido was an adjective. Shouldn't it be the adverb rapidamente here?

You could absolutely say quão rapidamente, and it would be perfectly correct. However, in modern Portuguese, it is very common to use the masculine singular form of certain adjectives (like rápido, lento, and duro) as adverbs. Because it acts as an adverb describing how they retain the image, it stays masculine and singular regardless of who is doing the action.

Why does the Portuguese sentence use conseguem instead of podem for "they can"?