Nerd Alert

(Warning: This is a pretty geeky and technical entry, but you may find interest if you are a language nerd like myself.)

Let's say you're at a party and you see two people, one pretty drunk. Maybe he's being very loud. Suppose someone says to him that he needs to lower his voice. Then he turns to his friend and asks if he's being loud.

If the friend says Habla como quieras.
This means 'Talk however you want.' However there is an indication that the friend does not know exactly how he will be acting. There is a degree of uncertainty.

But if the friend says Habla como quieres.
This also means 'Talk however you want.'  However there is an indication that the friend knows how he is going to talk and is suggesting that he is always loud and obnoxious when he is drunk.

What's the difference between the two? Only that 'a' or 'e'  on the root word 'quier'.
This is the use of something called the subjunctive mood. This is something that we have in english also, however we don't use it as widely. Here's an example in english.

You are happy. (indicative mood)
I wish you were happy. (subjunctive mood)

The were in the second sentence is the subjunctive. In English we use the past tense to make the subjunctive. Why the were? We're not talking about the past after all.  It's because it's a situation that suggests desire or a hypothetical situation, ie. I wish I knew spanish fluently, if I did I wouldn't sound like a clown.

But in spanish the subjunctive is used extensively, plus it's conjugated in its own tense, not the past tense as in english. This makes it very difficult for us, because when we talk about any situation that involves emotion or desire or hypothetical situations we have to remember to insert the special tense, not the Spanish past tense.

Further it's used to distinguish degrees of certainty. The 3 sentences below would all be translated in the same way in English. "We will have been here for 4 months when we leave."

Habremos estado aqui 4 meses cuando salgamos.
Habremos estado aqui 4 meses cuando salimos.
Habremos estado aqui 4 meses cuando saldrémos.

Why the different endings of the verb salir (to leave)? The sentence at the top means, we mean to leave but who knows what the universe plans. The one in the middle means we will definitely be leaving.The one on the bottom means we are positively sure and confident that we are leaving, so you'd better not suggest that we're not. You can see where things can get confusing.

Spanish is a bit more explicit in situations, whereas in English you have to read between the lines more. This can get confusing, because the Mexican culture is more indirect than the American culture. This is all a bit paradoxical. Basically it calls for a lot of forethought before you ever even begin to decide what word you are going to use. You need to decide what mood you are going to use.