Learn French: Le château est beaucoup plus imposant que nous ne pensions. - The castle is much more imposing than we thought.

This is the ne explétif (expletive ne). It doesn't make the sentence negative. In written or elegant French, it is often added after affirmative comparisons of inequality (like plus... que or moins... que). It highlights the gap between the reality of the castle and your expectation of it. You simply ignore it when translating into English.

Why use the imperfect tense pensions instead of the passé composé avons pensé?

The imperfect is used because your expectation or belief was an ongoing state of mind leading up to the moment you actually saw the castle. The passé composé (avons pensé) would sound like a single, isolated thought that started and finished in the past.

In English, we often say "than we thought it was". Can I add le to the French sentence?

Yes, you can! It is perfectly correct to say que nous ne le pensions, where the neutral pronoun le replaces the whole idea of "it being imposing". However, leaving it out (as in this sentence) is also very common and sounds totally natural.

Why is it beaucoup plus and not très plus for "much more"?