"¿Cuándo?" as an Interrogative Adverb in Spanish
The word "¿cuándo?" means "when?". It is used to ask questions relating to the time of events. We can call it a question-word question, in the sense that it is often used at the beginning of a sentence to form a question. We should also note that "cuando", without an accent, is a conjunction. Because we are explaining these words in terms of their function, we will save that case for another time. For our purposes, let's focus on this word as an "interrogative adverb". This word should be placed directly before the verb in a sentence. The word order for questions with the word "cuándo" is often the same as those for "dónde" and "qué".
[Interrogative Adverb] + [Verb] + [Noun]
¿[Cuándo] + [viene] + [Marcos]?
When is Marcos coming?
If we translate the question word for word, we get something like this: [When] + [does come / is coming / comes] + [Marcos]
The word order for this question is not entirely inflexible, but this order is a good model to follow, before learning to emphasize different parts of the sentence. Also, if the subject of the question is mentioned before the question is asked, then the subject is often omitted. For example, if I say "David va a venir" (David is going to come), then the question would be formed:
[Interrogative Adverb] + [Verb] ¿[Cuándo] + [viene]?
When is he coming?
Remember that the subject, once it is mentioned, is then indicated by the conjugation of the verb (in this case "viene"). While we know that this "viene" could be "he's coming", "she's coming" or "you're coming", it's understood that we're talking about "he" since the previous sentence mentions "David". Moreover, learning to recognize implicit subjects is essential to learning Spanish, since we tend to omit them unless there is ambiguity; and this ambiguity is usually brought to one's attention after the subject has been omitted!
AS AN INTERROGATIVE ADVERB:
¿Cuándo es la fiesta? (When is the party?)
¿Cuándo vamos a la playa? (When are we going to the beach?)
¿Cuándo quieres almorzar? (When do you want to eat lunch?)
¿Cuándo podéis ir? (When can you all go?)
¿Cuándo bailarás conmigo? (When will you dance with me?)
¿Cuándo te acuestas? (When do you go to bed?)
¿Cuándo te llamó Adriana? (When did Adriana call you?)
AS A CONJUNCTION:
Cuando bailo, me siento libre. (When I dance, I feel free.)
No me gusta cuando me miras así. (I don't like it when you look at me like that.)
Let's briefly note "cuando" and it's function as a conjunction. As such, it links main and subordinate phrases and joins words or sequences. "Cuando" is a temporal conjunction, and in this way, it expresses the temporal relation of the grammatical elements.
Consider the following examples:
Estaré en la oficina cuando llegues a la casa. (I will be at the office, when you get home.)
Cuando vayas al mercado, me compras unas manzanas, por favor. (When you go to the market, pick some apples up for me, please.)
Aside from its syntactic position, the main way to distinguish the usage is by the graphic accent (i.e. "cuando" = conjunction; "cuándo" = interrogative adverb) and by the accentuation on the first syllable, when it is an interrogative pronoun.