Hello! I have this sentence with dativ "Ist euch vielleicht schwindelig? Nein, uns ist nicht schwindelig". I don't understand why we use "ist" instead of the regular conjugation for the verd (wir - sind). Could someone explain me?
The key is the implied (es), that is actually missing from the sentence. The subject of the sentence is "es" (it) a singular pronoun with the conjugated verb form "ist", while "uns" or "mir" is the dative object. The dative object here stands for the audience/receiver of a feeling. The actual subject "es" confers no information at all. It simply fills the position of the subject similar to sentences like "It is cold." Who is this it? In some cases this information-empty "es" can be dropped. The result is a sentence without a subject, which is a rarity in German or English, but quite common in other languges like Spanish or Italian.
It's just one of these idiomatic things, I'm afraid.
It follows exactly the same (dative) pattern as Mir ist schlecht = I'm feeling sick/unwell; Mir ist nicht gut = I'm not feeling well etc - and these are common, everyday expressions you will hear all the time.
In the case of feeling dizzy, "schwindlig sein" never occurs in the nominative, i.e. "Er ist schwindlig" is wrong - it has to be Ihm ist schwindlig (dative). However: Er ist schlecht = He is bad (i.e. a bad person) vs. Ihm ist schlecht = He's feeling bad. So, in some cases, both nominative and dative constructions exist, and it's pretty crucial to know the difference.