1. Good. Either with or without the second "to" 2.It improves students' creativity, and they will end up with higher grades. GOOD 3.Air pollution is an urgent concern and the reason lies on fossil fuels. "The reason lies" is a bit unusual but correct. I would say "the reason can be found in ...." or the "blames lies with...the excessive burning of fossil fuels". The reason is not fossil fuels but the excessive burning of fossil fuels. 4. "to benefit a skill" is not obviously natural. You use a skill and this brings a benefit to a person or thing. 5. weak economy / strong economy: A lack of experts leads to a weak economy. Correct with "A" but it would be a naive statement from a native-speaker with a high-level education. You could think about trying to say something more meaningful. 6.many students struggle with their lessons.Their weakness lies IN their approach. GOOD

3.Air pollution is an urgent concern and the reason lies on fossil fuels - I would say - lies with fossil fuels 6.many students struggle with their lessons.Their weakness lies in (or lies on ?) their approach. - I would say lies in or lies with their approach The rest of them are fine as far as I can see Well done Hope this helps Jane :)