"What does he do with this?"
Translation:Wat doet hij hiermee?
The jij/je form of the present tense is the stem of the verb, 'doe' in this case, with or without an extra -t. The 't' is added when jij/je comes BEFORE the verb (e.g. jij doet het = you do it), but dropped when jij/je comes AFTER the verb (e.g. Wat doe je? = what do you do)
With the 3rd person singular (hij, zie/ze, etc), the -t ending is compulsory. As to why, it just is - sorry! Languages are confusing sometimes :(