Naturally, you wish to be honest, but an honest negative evaluation can have consequences far beyond what you may intend. If you are uncomfortable writing positively, you might consider declining. International TAs, particularly from Asia or Europe, should know that references from those areas are usually far more understated than those written in the United States. If you do write the recommendation, find out why the student has requested it, so that you can tailor your approach. Be as specific as possible, and use examples.
Omissions tend to be construed as negative.
Make at least brief mention of your student's academic skills, motivation and commitment, and interpersonal skills. If you say nothing of the student's personality, readers may conclude that he or she is a brilliant student but has difficulty talking to others. Avoid explicitly negative statements, no matter how many disclaimers are attached. CPPS will not show confidential recommendations to students. On occasion a counselor who notices a negative recommendation in a student's file may call to ask whether the impression given was the one that was intended.