Postby thegooseking » May 15th, 2017 10:05 pm
vasi_mahmudovaさん、
に is correct to use with 先週. There are (for the purposes of this discussion) two different types of particle in Japanese - case particles, and non-case particles. We don't use case very much in English (except for pronouns), but in other languages, the case is the way of marking whether something is the subject, object, indirect object, possessive, etc. Japanese has more cases than any other language I know of other than Finnish, but fortunately it represents case just by adding a particle rather than changing the word itself, so it's easier. So case particles express the relationship between the element they mark and the predicate (in this instance the predicate is the verb of the relative clause, 見た), while non-case particles have other functions.
に is a case particle. It represents the dative (indirect object) case, as well as the temporal case (which is how it is working here), the physical locative case for existential predicates (いる, ある and the copula), and the adverbial case where a noun (or な-adjective) is used as an adverb.
は is a non-case particle. It marks the topic of the sentence, but it doesn't really say anything about the relationship between what it marks and the predicate. In fact, non-case particles like は and も often 'hide' case particles. When you use は with a time-based noun, there is often an implied に particle "hiding behind" the は. For other sentences, there might be another case particle "hiding behind" the は, often the nominative (subject) particle が.
As for why we don't use は here, I'd say it's because it's part of the relative clause. If we used は it would be "Last week, who was the person who saw the movie?" You're asking "who was that person last week?" but presumably they are the same person this week, so it doesn't make sense to put a time constraint on who the person is. However, it does make sense to put a time constraint on when the movie was seen, so we use に to get "Who is the person who saw the movie last week?"
I hope that isn't too technical an answer.
小狼