Complex sentence: He joined the youth team of Yee Hope and Hong Kong Rangers when he was in secondary school, but had chosen to give up his dreams on playing football and instead concentrated on studying due to the pressure of handling both sides.
Split into simpler sentences: He joined the youth team of Yee Hope and Hong Kong Rangers when he was in secondary school. But he has chosen to give up his dream on football and concentrated on study due to the pressure of handling both sides.