Learn Italian: Gli attivisti hanno organizzato uno sciopero.

Words ending in -ista (like attivista or turista) are the same for both masculine and feminine in the singular. However, they have distinct plurals! A group of male or mixed-gender activists is gli attivisti, while a group of exclusively female activists is le attiviste.

Why is the article uno used instead of un before sciopero?

Because sciopero starts with an s followed by another consonant (c). Masculine nouns starting with s plus a consonant (as well as z, ps, pn, and gn) take uno instead of un. If we wanted to say 'the strike', it would be lo sciopero.

Where does the stress fall when pronouncing sciopero?

The stress falls on the very first syllable: SCIÒ-pe-ro. The sci makes an English 'sh' sound. It is a common mistake for learners to stress the second syllable, but this is a word where the natural stress is on the third-to-last syllable.

Since gli attivisti is plural, why doesn't the past participle change to organizzati?