Complex sentence: The people who have been imprisoned over the past 40 years have mostly been linked to this armed struggle and have been convicted of terrorist activities, or for belonging to or having belonged to ETA or other organisations linked to it.
Split into simpler sentences: The people who have been imprisoned over the past 40 years have mostly been linked to this armed struggle. Most of these prisoners have been convicted of terrorist activities, or for belonging to or having belonged to ETA or other organisations linked to it.