I am having difficulty perceiving where to use the "do". Yes, I know it is used in interrogative and negative sentences and therefore if used in them, it must also be used in the affirmative, but is this auxiliary used in all interrogative and negative sentences?
Or there are some expression without the "do"

See a translation

is do used in all interrogative sentences? = no

there are some where it's not used.
INTERROGATIVE
object questions: use "do"
subject questions: without "do"

eg.
affirmative: "The boy ate the cake"
boy = subject
cake = object

-What DID the boy eat?
(^object question)
- Cake

-Who ate the cake?
(^subject question)
- The boy

"Do" deve venire prima del soggetto della frase ma se il soggetto è una parola tipo:[what, who, ecc] non si mette do. In pratica Il soggetto del verbo deve venire direttamente prima del verbo, se il soggetto è proprio la parola della domanda (ossia what, who, ecc.) non si usa "do".

EMPHATIC INTERROGATIVE
"The boy ate what?!!"
this is a special interrogativo form used for surprise