improved grasp of the English language than a year ago

He showed a much improved grasp of the English language than a year ago.

This sentence is supposed to be grammatically wrong (the usage of the phrase "than a year ago"). Can you please explain to me why?

The sentence is wrong because improved is not a comparative.

He showed a much better grasp of the English language than [he showed] a year ago.

Or

He showed a much improved grasp of the English language compared to/over what he showed a year ago.

The [(what) he showed] bits can be omitted, and the sentence will still be grammatical, but also rather awkward as the structure would not be exactly parallel (see @Sandeep's answer); I recommend to include them