Answers:
`1)` in/we/ study / past/and/ present/ in/ events / vietnam/ and in / the / world/ history.
$\to$We study history events in past and present in Vietnam and in the world.
`2)` You/it/ have/ this/ book/ I/ when/ can/ finish.
$\to$You can have this book when I finish it.
`3)` English / English magazines/ by/ reading/ learns/ to /read/ Tuan/.
$\to$Tuan learns to read English by reading English magazines.