Questions & Answers about Du borde vara artig.
Why is there no att (to) before vara?
Because borde (should) is a modal verb. In Swedish, just like in English, modal verbs are followed directly by the bare infinitive of the next verb. So you say borde vara (should be), never borde att vara.
Doesn't borde end in "-de" like a past tense verb?
Yes, it does! Borde is technically the past tense of bör (ought to), but it is used exactly like "should" in English to give advice in the present or future. It sounds softer and more polite than telling someone they have to do something using måste (must).
Why do we use artig here instead of artigt or artiga?
The adjective has to match the subject it describes. Here, the subject is du (you, singular). Because a person is an en-word, we use the basic form artig. If the subject were plural, like ni (you all), it would become Ni borde vara artiga.
Where would I put a word like alltid (always) in this sentence?
Short adverbs like alltid usually go right after the first verb in a main clause. So you would say Du borde alltid vara artig (You should always be polite).