If this, then that

Many students have trouble with "if" clauses. Here's a brief rundown of the most commonly used clauses.
***Note that "si," when it means "if," does not carry an accent!

Si (presente del indicativo) + presente del indicativo, futuro, o mandato

• Si estás aburrida, es bueno leer.  If you're bored, it's good to read.
  
• Si quieres mi ayuda, pídela.  If you want my help, ask for it.
  
• Si tienes frío, ponte una chaqueta.  If you're cold, put on a jacket.
  
• Si no estudias, tendrás malas notas.  If you don't study, you will have bad grades.
  
• Si practicas mucho el violín, vas a tocar muy bien en el concierto.  If you practice the violin a lot, you will play well in the concert.
  

Si (imperfecto del indicativo) + imperfecto del indicativo
(This is used to refer to habitual actions in the past)

• Cuando era chica, si quería helados mi abuelo me los compraba.  When I was little, if I wanted ice cream my grandfather would buy it for me.
  
• Si Manuel estaba enfermo, su mamá le hacía sopa.  If Manuel was sick, his mom would make him soup.
  
• Si Marta tenía mucho trabajo, no jugaba.  If Marta had too much work, she didn't play.
  

Si (imperfecto del subjuntivo) + condicional
(This is used for hypothetical situations in the present)

• Si tuviera el dinero, viajaría a la China.  If I had the money, I would travel to China.
• Si fuera más alto, jugaría basket.  If I were taller, I would play basketball.
• Si sacara mejores notas, su mamá le compraría un regalo.  If he got better grades, his mom would buy him a present.

Si (pluscuamperfecto del subjuntivo) + condicional perfecto
(This is a more advanced form--if you haven't yet done the perfect tenses, don't practice this yet--this is for hypothetical situations in the past)

• Si hubiera sabido que venías, te habría preparado la cena.  If I had known you were coming, I would have prepared you some dinner.
• Si hubieras visto mi gol, te habrías impresionado.  If you had seen my goal, you would have been impressed.
• Si hubiera tenido más tiempo, habría estudiado otro idioma.  If I had had more time I would have studied another language.

Para practicar:

• http://www.colby.edu/~bknelson/SLC/si_clauses.php
  
• http://www.quia.com/cm/107395.html
  
• http://www.quia.com/cm/335306.html
  
• http://www.quia.com/cm/107396.html
  
• http://www.quia.com/cm/524778.html
  
• http://www.quia.com/cz/415912.html
  
• http://www.indiana.edu/~spangram/VERBFORMS/s-i-si.htm