to learning

[Over the top]

Be consistent because consistency is the key to learning a language.

Why is there -ing after to and not infinitive form? What is the difference? Is it incorrect to say consistency is the key to learn a language.

Thank you

[probus]

Be consistent because consistency is the key to learning a language.

Why is there -ing after to and not infinitive form? What is the difference? Is it incorrect to say consistency is the key to learn a language.

Thank you

The original is correct and your proposed alternative is not.

Consider "the key to the door" for example. "To the door" is an adjective phrase modifying "key".

Similarly in "the key to learning a language", "to learning a language" is an adjective phrase.

It consists of the preposition "to" and the gerund phrase "learning a language" which serves as the object. There is no infinitive involved.

If this does not help, study more about gerunds and gerund phrases.