In each pair of sentences, there is one correct and one incorrect sentences.

1. Julie realised she'd lost her key. Julie realised she lost her key.
2. Tom looks happy. Maybe he's had good news, or it must be his birthday. Tom looks happy. Maybe he's had good news, or it might be his birthday.
3. I was cycling to school when I saw Harry. I was cycling to school when I was seeing Harry.
4. Danny only set off ten minutes ago, so he can't arrive yet. Danny only set off ten minutes ago, so he can't have arrived yet