Hello Kseniya, I think that these is one of those slightly grey areas of English. But you are right they are both completed actions in the past. When you used 'once', it means a single specific occasion that you are referring to whereas if you use the present perfect it means a single occasion in your lifetime generally. Once you reach a certain level of English your choice of language depends on the message you want to convey. Hope this helps Bob

I think this is a good example of how word order can change meaning in English. What difference can you see, based on the position of "once" in the sentence?

I once witnessed a murder, but...> you witness a murder in the past ( past tense) I HAVE BEEN to England once. > You have until NOW been to England only once ( Present perfect tense meaning from the past until now ) Yes, both relate to a past experience , but past tense just refer to a time in the past , and present perfect refer to that experience happening in the period between the past and the present)

The difference is not so much caused by the word order or the time (past) as by the meaning of "once". In the 1st sentence "onces" means "some time ago" = однажды (ранее) In the 2nd sentence it means NUMBER (how many times), that's why the word order is the result of the meaning of "once".