Hello Federica, If you use 'once' you are talking about a specific occasion so it is a finished action in finished time so you should use past simple Once I went to a concert. You can use the sentence without once to mean 'in your life experience' I have been to a concert You can finish either sentence with 'which I really liked', this will make your sentence more natural. Hope this helps Bob

Can I use "that "instead of "which"?