You don't know English, so you have to study it in our school.
If you knew  English, you didn't have to study it in our school.
3. I think you should spend more time improving your English pronunciation.
If I were you ,  i would  spend more time improving your English pronunciation.
5. The foreign company turns down her application since she doesn't speak English well.
If she spoke English well,  The foreign company would turn down her application
6. Those tourists want to order a Vietnamese dish but they don't know Vietnamese.
If those tourists knew Vietnamese, they would order a Vietnamese dish