When you're trying to give good feedback, you can often negate your efforts by adding a "but" after a positive statement. By using "and" instead, you can avoid someone getting defensive or vulnerable.
Bill Gross, the founder and CEO of Idealab, explains that learning how to give good feedback takes hard work on his LinkedIn blog. Use of the word "but" is an easy first step you can take:
When you start telling someone, "you are really great at x, but when you do y..." the BUT negates all the goodwill that you are building up with the first part of your sentence. The BUT gets someone's defenses up, and makes them way less able to hear the important thing you want them to listen to. Instead, if you learn how to - and it sometimes takes hard work - craft your feedback with an AND, you can be MUCH more successful.
The word "and" changes everything. You're not telling someone there's a hidden catch to their ability or skill, just that there's more to the point you're trying to make. Instead of "you are good at x, but you're bad at y", it can be "you are good at x, and if you keep working at y, you'll be even better." Feedback should be constructive, so keep it short and sweet, and avoid bringing down the people you're trying to help.