To complete these sentences correctly, we'll decide whether to use 'than' for making comparisons or 'not' for negations.
The blue car is faster than the red car.
We use 'than' here because it's making a comparison between the speeds of the blue car and the red car.
Peter is not as tall as Rohit.
In this sentence, 'not' is used to show negation - indicating Peter's height is less than Rohit's height.
His copy is worse than the other one.
'Than' is used since this sentence is comparing the quality of his copy with another one; we use 'worse' because it's the comparative form of 'bad'.
The tomato soup was more delicious than the mushroom soup.
'Than' is used because it compares the tastes of two types of soups, with 'more delicious' (or simply 'better') showing the comparison.
Silver is not as costly as gold.
'Not' is used to say that silver costs less than gold.