Question

mark _____ run fast two years ago but now he can .​

Answer 1

Answer:

used to

Explanation:

Mark used to run fast two years ago but now he can5

Answer 2

Mark couldn't run fast two years ago but now he can.

The sentence states that the person, Mark is now able to do what he wasn't able to do in the past, which is run fast. We can clearly see that the first part of the sentence before the conjunction is in the past tense and the second part is in the present tense. So, we use the past form of the most suitable word here.  

Additionally, in the second part of the sentence, the word 'can' has been used, which makes it most suitable to use its negative past tense form, 'couldn't'.