“Do” is a curious word in English. As well as forming half of the “do” vs “make” conundrum (“hacer” in Spanish covers both English verbs), it also appears in places which are quite strange to Spanish speakers, such as questions. For example:
¿Qué quieres? – What do you want?
In this case, “do” does absolutely nothing. “What you want?” makes perfect sense, but it’s just incorrect. And this construction is used in questions using “why”, “where”, “what/which” and “how”, and questions that require a yes/no answer. However, it is not used with any form of the verb “to be”.
A few more examples:
Do you like soup? (yes/no)
Why do you like soup?
What soup do you like?
How do you make soup?
If you are interested in the technical term, it’s called an auxiliary verb.
If you ask questions in the past, “do” is still used, but in the past tense. Note that the other verb remains in its simplest, present form.
¿Qué viste? – What did you see? (NOT “What do you saw?”)
¿Cómo viajaron? – How did they travel?
“Do” also appears in negative statements and follows the same rule. Consider these examples:
I know / I don’t know (present tense – “do” is in present tense)
I did it / I didn’t do it (past tense) – “do” is in past tense)
She cares / She doesn’t care (present tense – “do” is in present tense)
You’ll start to see variants of “do” everywhere now. But you love it, don’t you?