No, you can use the emphatic versions in many situations, not just when contrasting with Ik, but if there is no purpose to the emphasis, then zij/jij sounds a bit formal. Ga jij naar de stad? is fine, but Ga je naar de stad? sounds more normal and relaxed.
Duolingo puts great explanations in the Tips. The Tips show up when you click on a Skill. Many Skills (but not all) have these Tips. Always read the explanations before doing the lessons.
It was translate sentence exercise in Stress pronouns... yes the answer is correct but it kept telling me was incorrect... think it must be a glitch in the application... I triple checked against the correct answer... and this module refuses to go gold... just did it again... and got through successfully this time... but the module remains green not gold...