1. HelloTalk으로 썼고 많은 사람들과 얘기했는데 이제 안 사용한다 2. HelloTalk을 썼고 많은 사람들과 얘기했는데 지금은 사용하지 않는다. To me, both sentences sound correct, with a slight nuance difference. (을 is better than 으로 in this context, although 으로 is not necessarily wrong) 이제 often implies some sort of connection with the past, like because of or after what happened before, "now I ...". So #1 sounds like the reason you're not using it anymore is because you had a (negative) experience with it. 지금 doesn't have such a nuance, so #2 means it just so happens you don't use it now, with nothing implied. So depending on what you want to express, I think either #1 or #2 can be said. As for 지금 vs 지금은, 지금 sounds much worse in here, because 은 gives the sense of separation from the past. In #1 however, 이제 and 이제는 don't sound that different. It's hard to explain why, but that's how it sounds. As for the second question, counters for people are only necessary when you count them explicitly. Just because there are more than one person, it doesn't mean you must use the counter 명 or 분 (honorific of 명). 두 사람 sounds more natural than 두 명 or 두 분 when the number of people is not the main focus Also, we try not to use pronouns or count words for people if possible - normal full words show more respect. For things and animals, count words are more frequently used. In Korean we also use 두 or 둘 a lot more than in English. For example, 두 눈을 감다, 두 손 모아 빌었다, etc, for which one would just use a plural without "two" in English. It's partly because pluralized nouns aren't used as much as in English. You can think of it as a custom in a language.

는/은 which is used after 오늘, 지금, 내일 is a topic marker. it emphasizes the time of 오늘, 지금, 내일. 지금 사용하지 않는다. I don't use now. 지금은 사용하지 않는다. I don't use *now* = I don't use recently. 사람 is a noun and it can be used a counter as well 한 사람(a person) 남자 두 사람(2 mans) 어린이 다섯 사람(5 children) 학생 세 사람(3 students) Refer to http://www.koreanwikiproject.com/wiki/Counting_items