Juan likes coffee? Don't forget the do!
The correct is: "Does Juan like coffee?"
Do you like coffee? Do you like tea? What do you like to do? Where do you live? Where do you come from? Where does he come from?
These are all questions.
Now, let's see some negatives.
I don't like coffee. I don't like tea. I don't know. And so on!
What do these questions have in common?
They all have the auxiliary verb "do".
If I had a dollar for every missing "do" in a question… But hey, it's not completely your fault. I know, in Portuguese, changing from interrogative [?] to affirmative [+] is easy. All we gotta do is change the way we pronounce. It's cool. But let's make of today, the day that you finally learn this: we use do or does for questions and negative sentences in the simple present tense!

Louder: we use "Do/Does" for questions and negative sentences in the simple present tense!
In English, you can say that you have two types of question: Yes/No questions, and WH questions. Today we are talking about the first type. That is: those questions in which the answers can only be: Yes, I do or No, I do not.
So, all you have to do is remember that you need to use DO/DOES + VERB in the base form in order to make the question. Check this:


Think like this:
hmmm… Let's see. I want to know if Juan likes coffee… Easy. Let's see. Juan is a man. Juan is alone (singular), so… he!
Ok. Now, I am asking a question! So… I need to start with does — because we are talking about Juan! Amazing. So… "Does he…".
Now what? Oh, of course: the verb now! I want to know if he likes coffee! So… like!
"Does Juan like…" What now? Oh yes! Coffee! Got it.
Does Juan like coffee?
Se você leu até agora e entendeu, ótimo. Se você não entendeu, it's ok. Long story short, é o seguinte:
A importância do verbo auxiliar é subestimada. Ele é muito importante: o verbo auxiliar "do" é o verbo que nos mostra se a frase em questão é uma pergunta (de acordo com a ordem) ou negação (do + not), e qual o tempo verbal (simple present "do", simple past se você colocar did — passado de do).
No simple present, nós usamos o do/does para perguntas e negações. Do para I/you/we/they. Nós usamos o does para a terceira pessoa: he/she/it. Nas perguntas, ele vem antes do sujeito. Ex: Does Juan (he) like coffee? Nas negações, vem depois, e acompanhado do not. Ex: Juan does (he) not like coffee; or Juan doesn't like coffee!
It's that simple.
What do you think?