When the Main clause expresses uncertainty or doubt, the verb in the subordinate clause is in the subjunctive

"Dudo que yo pueda estar ahi a es hour"

"I doubt that I can be there at that time"

When the main clause denies or negates what is expressed in the subordinate clause the subjunctive is used.

Ana niega que Carlos sea su novio

"Ana Denies that Carlos is her boyfriend"

The verb creer is followed by the subjuntive in negative sentences , where it express disbelief

"Teresa va al hospital?"

"No, No creo que vaya hoy"

Is Teresa going to the hospital today?

No, I dont think shes going today

Refers to a future action or is a command. Some Conjunctions of time are:

Cuando(when)

Tan pronto como& en cuanto(as soon as)

Hasta Que (until)

En Cuanto ( as soon as) 

No, Vamos a esperar hasta que venga Eva (No, we're going to wait until Eva comes)

A menos que (unless)

Antes de que (before)

En caso de que (in case)

Con tal (de) que (provided that) 

Para que ( in order that, so that)

Sin que (without)

1) By using the first person plural of the present subjunctive

preguntemos el precio de la habitación

" Let's ask the price of the room"

2) By using the expression vamos a + infinitive

Vamos a preguntar el precio de la habitacion

"Let's ask the price of the room"

The verb ir does not use the subjunctive form in the first pserson plural affirmative command

Vamos al teatro

But in a negative command , the subjunctive is used

No vayamos al teatro

are attached to the verb and a written accent is then placed on the stressed sylable

Compremoslo (let's buy it)

llamémoslos. (let's call them)

Usted

Hable

coma

abra

cierre

vaya

ustedes

Hablen 

coman

abran

cierren

vayan

Tu Command forms

Tu

Affirmative          Negative

Habla                no hables

come                 no comas

abre                 no abras

cierra                 no cierres

ve                     no vayas

Nosotros Command forms

Nosotros

Hablemos

Comamos

Abramos

cerremos

vamos

                                                (the negative uses no vayamos)

What translates as Que? when it is used as the subject of the verb and asks for a definition

Que es una paella?   What is a paella?

What? translates as cuál? When it is used as the subject of a verb and asks for a choice. Cual conveys the idea of selection from among several or many available objects, ideas etc

Cual es su numero de telefono?