[Simple Present] [Preterit] [Imperfect] [Future] [Conditional]
Use the simple present tense when speaking about what you usually do.
See conjugation of regular, irregular and stem-changing verbs.
Use the preterit when talking about something that has already happened, ...
 
... to denote an action which was performed and completed in the past. El Sr. Vegas llegó a la convención el martes. 
Mr. Vegas arrived at the convention on Tuesday.
... to express the beginning of a past action.  La clase comenzó a las 9:00. 
The class began at 9 o'clock.
... to express the end of the past action.  La convención terminó el viernes. 
The convention ended on Friday.
... to express an action which occurred over a specific period of time. La convención duró cuatro días. 
The convention lasted for four days.

See conjugation of regular, irregular and stem-changing verbs. Some verbs change their meaning when used in preterite -- see the list. Also see Preterit vs Imperfect.

Use the imperfect ...
 
... to describe actions or states in progress in the past in 
which neiter beginning nor the end matters.
Ayer a las cinco me bañaba.
... to describe two or more actions in progress in the past that 
occured simultaneously. Use mientras and y to connect them.
Yo tocaba guitarra y tú cantabas.
... to describe an action in progress in the past which
was interrupted by another action. Use cuando to connect them.
Mi novio me besaba cuando
mi padre abrió la puerta.
... to describe habitual actions in the past.
NOTE: Soler is never used in preterit, only in imperfect as it refers to habitual actions. In imperfect it can be translated as used to.
Todos los días me miraba en clase.

See conjugation of regular, irregular and stem-changing verbs.  Also see Preterit vs Imperfect.

Use the future when referring to an action that will take place in the future. In Spanish there are two ways to do it. When refering to a certain event in the near future, one uses present tense (Futuro Popular), identical to "I am going to" construction in English: ir + a + verb. Alterntively one can use future tense (Future Gramatical).
For formation of future tense see conjugation of regular and irregular verbs. There are no stem changes in future tense.
Use the conditional ...
... to express what someone would do
Sería interesante hacer un estudio ...
... to give advice using yo que tú/él/... and (yo) en tu/su lugar phrases
Yo que tú les diría verdad.
En tu lugar les diría verdad.
... to make polite requests.
¿Podrías decirme dónde está el correo?
... when discussing situations in the present or future that are contrary to the fact (2nd type of conditional sentences).
Si lloviera, no saldría.

For formation of conditional tense see conjugation of regular and irregular verbs. There are no stem changes in conditional tense.