Learn Portuguese (Portugal): Há dois dias que eles preparam o complô silenciosamente. - They have been silently preparing the plot for two days.

In Portuguese, when an action started in the past and is still happening right now, you typically use the present tense. This is combined with the structure Há [time] que (Literally: 'There are [time] that'). While English requires the complex present perfect continuous tense, Portuguese simply expresses this as 'There are two days that they prepare the plot.'

Could I rewrite the time expression using desde (since)?

Yes, but the grammar structure changes slightly. If you use desde, you drop the que. You would say Desde há dois dias, eles preparam o complô (Since two days ago, they prepare the plot). The version used in this sentence, Há dois dias que, is extremely natural for spoken Portuguese and places a slight emphasis on the duration itself.

Since this lesson focuses on the spatial adverbs cá and lá, could I use cá in this sentence to add pragmatic focus?

Absolutely. You could say ...que eles cá preparam o complô silenciosamente. Even if they aren't physically 'here' in the room with you, adding cá gives a pragmatic focus. It implies the speaker feels highly involved in or aware of this ongoing situation, treating the plotting as a present, pressing reality rather than a distant fact.

Is complô a common word, or would native speakers prefer conspiração?