Start with praise
It’s easier to hear something negative after we are praised for what we have done well. Be sincere about your honest appreciation for the other person and their hard work to soften the blow, and to avoid coming off as though you’re reprimanding them. Try to avoid the word “but” after the praise and instead use the word “and.” For example, “We’re really proud of you for raising your grades, and if you keep up the good work, you can get your algebra grade up as well.”
Don’t make it personal
Make your criticism about work or behavior you’d like to see change, not about personality or personal attributes. Give criticism without being critical of the other person and recognize barriers that might be in their way. Follow up with offering up why it’ll be beneficial to the other person to change their behavior. For example, “I know we have a crazy schedule. Try to stick to your deadlines next time to avoid making your workload even heavier the following week,” will be much better received than saying, “you’re too slow and need to keep up with your deadlines.” These phrases, on the other hand, can make any argument worse.