Choose the right form of the verb: One needs practice when _________ a foreign language.

The correct answer is learning.

Explanation: The sentence requires a gerund (verb + -ing) after the preposition 'when'. The gerund 'learning' functions as a noun in this context, describing the activity one needs practice in.

Here's why the other options are incorrect:

• to learn: This is an infinitive form, which is not appropriate after 'when' in this sentence structure.
• having learnt: This is the past participle, which is used in perfect tenses and doesn't fit the context here.
• will learn: This is the future tense, which doesn't logically connect with needing practice in the present.

Therefore, learning is the most grammatically correct and contextually appropriate choice.