Hiya! I would watch out with translating "on" from English to "op" in Dutch. It only means "op" if it's referring to a point in space. Example: The chair is on the table. Becomes "De stoel is op de tafel". That's pretty much the only use of "on" meaning "op" in Dutch. For that sentence, "I need to work on replies" I would translate it as "Ik moet werken aan antwoorden." The verb "work" (as you probably will have noticed) is not singular. In this sentence, "need" ("moet") is the main verb, and is therefore conjugated in the first person singular (I, "ik"). The verb "work" ("werken") is used as the infinitive: to work (werken). The "to" and "work" belong together. There is no such construction in Dutch, though. All verbs in Dutch end in "-en" in their unconjugated form. ..if you have any more questions about this horribly confusing language, feel free to ask me :)