"You are drinking cold milk."
Translation:तुम ठंडा दूध पी रहे हो।
'tum' = 'you' and tumhe='to you'.
In this case, the Hindi sentence is a straightforward literal translation of the English sentence. So, 'tum'/'you' is used.
You might be confused with sentences like तुम्हें दूध पसंद है for 'You like milk'. In cases such as this, there is no Hindi verb that corresponds perfectly to the English verb. As a result, different phrasing is used. तुम्हें दूध पसंद है is literally something like 'To you, milk is liked' which uses the adjective पसंद because there is no Hindi verb corresponding to 'like'.