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Which is more correct:

My hobbies include learning different languages and also improving my English grammar skills.

My hobbies include learning different languages and also improving my English grammatical skills.
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Use grammar skills; that is the normal collocation. The second is odd if you think about it (probably just because it is not the usual choice): what kind of grammatical skills would one have? The ability to decline a noun, the ability to write subjects and verbs in concord, etc?
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AH020387 Which is more correct:
My hobbies include learning different languages and also improving my English grammar skills.
My hobbies include learning different languages and also improving my English grammatical skills.

I suggest this.

My hobbies include learning different languages and also improving my English grammar skills.

Students: We have free audio pronunciation exercises.

it is about the sense, english is tricky and amazing language

if we say grammar skills we are generally joining two nouns grammar + skills

so if u say grammar skills it is just combined of two noun where there is no other connection

but grammactical is about adjective and grammatical skills is a combination of both adjective and noun, where adjective is adding something in the meaning of noun

so 'grammatical skills' is about combination of two words which are grammactically interconnected