Hobbies (Hobby.)

I am fond of learning English. I am keen on foreign languages (.) and (I) have learnt them for 2 years. I like music (.) and (I) am fond of playing the piano. I am keen on swimming (.) and (I) have swum since childhood.

As you are referring to more than one hobby, you need to use the plural form 'hobbies'. Apart from your first sentence, the others can be paired up by using the conjunction 'and' in which case you don't need to repeat the subject (I).