Learn Portuguese (Portugal): Há dias que a mulher observava a coruja atentamente. - The woman had been observing the owl attentively for days.

You are right! While há alone often means 'there is' or 'ago' (like há dois dias meaning 'two days ago'), the specific structure Há [time] que... is used to express how long an action has been going on. In this context, it translates to 'For [time]'.

Why is the verb observava in the imperfect tense instead of a compound tense like the English 'had been observing'?

Portuguese rarely uses compound tenses for this kind of continuous past action. Instead, combining Há [time] que with a verb in the imperfect tense (observava) perfectly captures the idea of an action that was ongoing for a period of time in the past.

Could I put 'for days' at the end of the sentence instead of the beginning?

Yes! However, if you move the time expression to the end of the sentence, you must drop the word que. It would become: A mulher observava a coruja atentamente há dias. Both structures are very common and mean the exact same thing.

How is the word atentamente formed?

This is an adverb ending in -mente, which is the Portuguese equivalent of the English '-ly' (like 'attentively'). To build these adverbs, you always take the feminine singular form of the adjective (in this case, atenta) and attach -mente to the end.