Breakdown of Du måste vara försiktig på vägen.
vara
to be
du
you (singular)
måste
to have to, must
på
on / at / for (wait for) / in (believe in)
Questions & Answers about Du måste vara försiktig på vägen.
Why do we use vara (to be) here instead of är (am/is/are)?
Måste (must) is a modal verb. In Swedish, just like in English, a modal verb is followed by the infinitive (dictionary) form of the next verb. Therefore, we use the infinitive vara instead of the present tense är.
Why is the adjective försiktig and not försiktigt or försiktiga?
The adjective must match the pronoun it describes, which is du (you). Since a person is a common gender ("en") word and we are only talking to one person, we use the basic singular form försiktig. If you were speaking to multiple people (ni), you would use the plural form försiktiga.
Could I say i vägen instead of på vägen?
No, that would completely change the meaning! While på vägen literally means "on the road" (driving or traveling), the phrase i vägen is an expression that means "in the way" (as in, blocking someone or something).
How might I use the conjunction så (so) with a sentence like this?