तुम is grammatically plural even if you are using it to refer to a singular subject. Just as in French Vous is grammatically plural even when using it to refer to a singular subject. English does something similar in using what is referred to as "The Royal We", but much less often, in formal conversation "We" can be used to mean "I".
तुम also always conjugates with ए (तुम जा रहे हो, तुम जाते हो, तुम खाते थे, etc.) even though it's singular.