1. I saw you as I was COMING OUT OF the cafe. (these happened at the same time: you could however say 'I saw you when I was at the cafe'.) 2. It started to rain just as I WAS LEAVING THE HOUSE. (or ...'I LEFT HOME') 3. As I didn't have enough money for a taxi, I DECIDED to walk. 4. Just as I took the picture, it BEGAN TO rain. I have this book and have looked at the chapter. Number 3 does make sense as you had it, but it would imply that you INTENDED to walk, but perhaps ended up not doing so.

Here are your sentences revised for naturalness and correctness: 1. I saw you as I was at the cafe. * This is mostly correct, but it would be more natural to say: "I saw you while I was at the cafe." 2. It started to rain just as I left home. * "Leave" should be changed to "left" to match the past tense. 3. As I didn't have enough money for a taxi, I decided to walk. * "Went to walk" should be changed to "decided to walk" for clarity. 4. Just as I took the picture, it started to rain. * "It was rain" should be changed to "it started to rain" for correctness and clarity. So the corrected sentences are: 1. I saw you while I was at the cafe. 2. It started to rain just as I left home. 3. As I didn't have enough money for a taxi, I decided to walk. 4. Just as I took the picture, it started to rain.

Have you got the "red cover" English in Use or the "violet cover" English in Use?