Both you and Naver translation did quite a good job. One of your questions is very easy. "다" is not only in dictionaries. In English dictionaries, for example, you can find verbs in their infinitive form, and you can also use that form in real sentences. <- I just used "use" in this sentence. And moreover, "아니었다" is not in original form. You can find "아니다" in Korean dictionary because it is the original form(원형) of 아니었다. 아니다 means "to be not", 아니었다 is past tense conjugation of 아니다. 게 is contraction for 것이 which breaks down into 것 + 이(subject marker). 것 means an action or a situation. 명 is used when you count the number of people. It is used like units, 3L of water, 2kg of meat, 5명 of 사람, something like that. You don't say 명 unless you are saying a certain number of people. You said "large crowd" not the exact number of people. 좀 is contraction for 조금. But I think it does not translates to "very". I often translate 조금 into "a little" or "a few". The trick here is that "아니었다" is denying being "조금". If 아니었다 were not present in the sentence, It would be "사람이 좀 많은 것이었다.", which translates to "It was a situation of being a little crowded". The translation "crowded" came from 사람 and 많은, each of which means people and to be many. And finally, 아니었다 is denying being 조금 here, so it was "not a little", so they combined can translate to "very" or "extremely". 사람(people)이(subject marker) 좀(a little) 많은(many) 게(the situation) 아니었다(not, past tense) Do you get it?