My hobby is swimming. I go to the swimming pool three times a week. I was four years old. There are six B long. My swimming teacher teaches D to swim well. He also tells us to be careful in water. I can already swim E than the other children in my group, but only a short distance. I am learning to swim on my back. Last week there was a competition and I won it.

The sentence structure and context indicate that the past tense is required for the first blank (A) as the action of going to the pool happened in the past. The word 'began' fits grammatically and contextually. For the second blank (B), the sentence refers to multiple children, so the plural possessive 'children's' is appropriate. For the third blank (C), the sentence describes a lesson that is ongoing or a general statement about lessons, making 'is' the correct verb. For the fourth blank (D), the teacher teaches 'us' as the object of the verb. For the fifth blank (E), the sentence compares the ability to swim, requiring the comparative form 'more quickly'.