`14`. It is the first time I have been to the ballet before
`→` It is the first time + (that) + S + have/has + Verb (PP)
`15`. It is twenty years since John and Mary last moved to Edinburgh
`→` it is... since S + V (Động từ V ở thì quá khứ đơn)
`16`. For two months now the police have been looking for him
`→` Hai tháng nay, cảnh sát đã tìm kiếm anh ta
`17`. This is the first time I have seen a dirtier-looking dog
`→` This is the first time + S + have/has + P2: Lần đầu làm gì
`→` S +have/has + never + P2 + before
`→` S+ have/has not + P2 + before
`18`. It is hours since Mary last rang
`→` it is... since S + V (Động từ V ở thì quá khứ đơn)
`19`. Alan hasn't visited his parents for ages
`→` S + hasn't/haven't + Ved/V3 + S
`20`. It is 8 years since she last started to learn English
`→` it is... since S + V (Động từ V ở thì quá khứ đơn)
`21`. It is the most beautiful building I have ever seen
`→` Đó là tòa nhà đẹp nhất mà tôi từng thấy
