Question

How old were you when you learnt ___? (drive)​

Answer 1

Answer:

How old were you when you learnt driving?

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Answer 2

How old were you when you learnt to drive?

Verb forms:

• A verb can either act as an infinitive or a gerund or a participle depending upon the nature of the sentence
• A gerund is a verb in the continuous or the -ing format in the sentence. For instance, I will be playing with you. The verb playing is continuous and also a gerund in nature.
• The participle form of the verb is the V3 type of any verb. It is the past of the past tense of a verb. Like the past participle for the verb in past tense 'took' would 'taken'.  
• Lastly, the infinitive is the to+verb form of any given verb. For example, "The boy has to eat that food". The verb 'to eat' is our infinitive.
• In the asked question a past participle won't be suitable, however, we could add a gerund or an infinitive after the past tense verb 'learnt'.
• The gerund for the verb 'drive' will be 'driving' and the infinitive for the verb 'drive' is 'to drive'.
• But, when we add the gerund in the blank, the tense of the sentence completely changes, so to maintain the same tense the infinitive form of the verb 'drive' will be added.

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