Rule: Infinitive clauses functioning as adverbials can only express purpose.
Examples of errors:
(1) One can only learn a colloquial language in the country where it is spoken, to be able to speak with the natives for a long time.
(2) Learning a foreign language begins to be able to read a label on a tomato can.
(3) If we had walked, we would have been home earlier than to go by car.
Infinitive clauses can function as noun modifiers (relative and appositive clauses), noun clauses, verb complements, and adverbial clauses of purpose, e.g.:
The man to see is Mr. Smith. (relative clause)
The need to see him is urgent. (appositive clause)
To know him is to love him. (nominal clause)
He's beginning to make progress. (verb complement)
He came to see me. (adverbial clause, infinitive of purpose)
Infinitives cannot function as adverbials of means, however, which is what is called for in (1) and (2). To be able in (2) could be mistaken for a verb complement, but this is not intended. (3) requires a conditional clause as the second element of the comparison, parallel to If we had walked.