Answer:

Who? – ¿Quién?

When asking about one person, you say ¿Quién? But when asking about more than one person, you have to change it to its plural form, which is ¿Quiénes? If you need to say “whose”, you would use de quién.

• ¿Quién es? – “Who is it?”

• ¿Quiénes son? – “Who are they?”

• ¿De quién son estos libros? – “Whose books are these?”

What? / Which – ¿Qué? / ¿Cuál?

In Spanish, “what” has two different words: qué and cuál. This can get a little confusing, but an easy way to remember it is that qué has only one factual answer possible. You use cuál to ask about a personal opinion or a choice between options. Cuál is a bit closer to “which” in some cases. Here are some examples:

• ¿Qué haces ahora? – “What are you doing right now?”

• ¿Qué es esto? – “What’s that?”

• ¿Cuál es tu favorito? – “Which is your favourite?”

• ¿Cuál es tu película favorita? – “What is your favourite movie?”

If asking about something that's plural, cuál becomes cuáles. Like in the last example, if you instead asked “What are your favourite movies?” it would be ¿Cuáles son tus películas favoritas?

When? – ¿Cuándo?

To ask “when” is easy: you say cuándo. The only thing to note here is you can’t use cuándo when you’re asking for a specific time (in hours and/or minutes). For that, the question is a qué hora as in ¿A qué hora es la película?: “What time is the movie?”  

Here are some examples with cuándo:

• ¿Cuándo es tu cumpleaños? – “When is your birthday?”

• ¿Cuándo vas a ir? – “When will you go?”

• Where? – ¿Dónde?

Dónde means “where”, but it has two other forms based on the preposition you use with it. A dónde means “to where”, while de dónde means “from where”.

• ¿Dónde vives? – “Where do you live?”

• ¿A dónde vas? – “To where are you going?”

• ¿De dónde eres? – “Where are you from?”

• Why? – ¿Por qué? / ¿Para qué?

Ah, the difficult por vs. para situation. It can be a bit difficult to know when to use which. It helps to think of para qué as “what for” or to understand the purpose of something. And think of por qué as “why”, or to understand the cause of something.

• ¿Por qué dices eso? – “Why do you say that?” (What caused you to say that?)

• ¿Para qué estás aprendiendo español? – “What are you learning Spanish for?” (For what reason or purpose?)

• How? – ¿Cómo?

Cómo is fairly straightforward, asking “how”. How someone is, how they do something, etc. But it doesn’t apply to “how much” or “how many” (I’ll come to that next).

• ¿Cómo está usted? – “How are you?”

• ¿Cómo hiciste eso? – “How did you do that?”