"He walks slower than you."
Translation:वह तुझसे धीरे चलता है ।
तुझसे is a form of तू and तुमसे is a form of तुम. In addition, we have आपसे which is a form of आप.
तू is the most informal form of 'you'. It is reserved for very close friends or family members who are of around the same age as yourself. It may be construed as rude if used for others.
तुम is a slightly less informal form of 'you' that can be used for most people of your own age or younger. It is also used to show intimacy.
आप is the formal form of 'you' that is used for people older than you and also in professional settings or with strangers when you want to be formal.
The verb चलता agrees in gender and number with the subject, which is वह. Since वह is singular, it needs to be वह तुझसे धीरे चलता है if the person you are talking about is male and वह तुझसे धीरे चलती है if they are female.
It's possible you are confused with the oblique case. Only the object of a postposition needs to be in the oblique case. The object is the part of the sentence which answers the question 'किस से?'. For example, in the sentence वह मेरे भाई से धीरे चलता है। (He walks slower than my brother), the object of से is मेरे भाई which is why we have used the oblique case form मेरे भाई instead of मेरा भाई.
In the sentence वह तुझसे धीरे चलता है, there is no postposition as such. For pronouns, instead of using a postposition like से, we use a special form of the pronoun. For तू it is तुझसे. So, you don't need to worry about the oblique case in this sentence.