Why does the imperative sometimes use "u/je"?
On the buses you often see "Vergeet u straks niet uit te checken", along with the English translation "Please remember to checkout".
But why the "u"? The imperative as taught here says you need to drop the subject, and "vergeten" isn't a reflexive verb, so what's going on?
In this case it is hard to see because "vergeten" already has a t at the end of its stem, but in these kind of imperative sentences that do have a subject added, the verb is not in the imperative form but in the regular present tense.
Sentences like this, that resemble the "true" imperative sentences (with the verb first), also have an urging/encouraging/strongly suggesting meaning, but the lack of a true imperative verb form gives them a slightly less commanding/rude tone. Therefore they are often used in these kinds of public notices as a more polite way to present instructions.
Let me give you another example, where you can see the verb difference:
"Stap hier uit ." = "Get off here" (e.g. off a bus, train, etc.)
"Stapt u hier uit." = "Please get off here."
Hope this helps!