서

Marks how many do it together

Attaches to a word for a number of people to say how many are doing the thing. Every one of these eight lines is the same word, 혼자서, 'on one's own' — eating ramen alone, practising alone, posting thirty messages by yourself. Plain 혼자 already means 'alone'; adding 서 puts the emphasis on carrying the activity out that way. Its partner is 이서, which does the same after an actual number.