This sentence is taken from (Cambridge Dictionary):
"He was improving for a while, but I think he's sliding back into his old habits."
I am asking why it is not in the present perfect tense (has improving).
Hello, Izzat Hanna,
"Has improving" is ungrammatical. The present perfect tense would be "has improved," and the present perfect progressive would be "has been improving." I think it is the present perfect progressive that you meant to use.
The sentence works with either the present perfect progressive or the past progressive in the first independent clause. I would find the present perfect progressive more desirable if there were only one independent clause:
He has been improving for a while but is sliding back into his old habits.
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