15 The Sun, the Moon, the planets, and the stars all rise in the east and set in the west.

16 I often learn foreign languages very fast but I experience some problems with Chinese at present.

17 Her private doctor still does not understand much about the disease.

18 The number of cars in Vietnam is increasing significantly these days.

15. The Sun, the Moon, the planets, and the stars all rise (rise) in the east and set (set) in the west.

16. I oftenlearn  (learn) foreign languages very fast but I experience (experience) some problems with Chinese at present.