When we are talking about conditions, we may think it’s something possible or imposible.

POSSIBLE (type 1)  present - will

In the if-clause you have a present tense and in the main-clause you have a future:
   
          If it rains, I will take my umbrella
          If you want, I will help you
 
   In both examples, the main clause expresses a possibility and what will happen in the future if it comes true

FANTASY (type 2)  past - would

In the if-clause you have a simple past and in the main-clause you have a conditional. In this case, you may be talking about the present or about the future, but you always consider the situation to be unreal.
 
future idea:
   1- If you marry me, I will make you happy    (=will you?)
   2- If you married me, I would make you happy (= but you won't)
 
present idea:
   3- If you lived in Nigeria you would hate the sun   (=but you don't)
   4- If I Knew it I would tell you    (=but I don't)
 
Examples 3 and 4 are clearly non-fact: I don't know it, so I can't tell you.
Example 2 is not impossible; after all, she might marry me, but the important thing is that I present my message as if I thought it is unreal, impossible (I think or I pretend to think you won't marry me), but in example 1, I present the same message as a fact, as a real possibility.


TRUTHS (type 0)  present - present

In English we can express a truth in the form of a condition. In this case, it’s not a hypothesis , but a fact, and we use a present tense in both sentences:
 
          If you heat ice, it melts
          If you look at her, she flushes
          If you are from France then you can speak French
 
In these cases we are not thinking of the future, but of the present. In the 1st example we are talking about a property of the ice (a present or timeless property) and in the 2nd example we are talking about a characteristic of her, not what will happen in the future, but what happens every time (habitual present). The 3rd example is even more obvious.
 
compare:  
        1- If you heat ice, it melts
        2- If you don't eat your ice-cream quickly, it will melt
 
In the first example we are thinking of a present property of ice, no one is probably thinking of heating it. In the 2nd example we are talking about the future, about what will happen if you don't eat it soon.
 
In type 0 conditionals, "if" can be replaced by "when" or "wheneve"r (=every time)
   If you drop a stone, it falls down = when / whenever you drop a stone, it falls down

UNLESS = if...not
The if-clause usually goes at the beginning, the unless-clause usually goes at the end
 
   Don't touch it unless you're going to buy it = If you are not going to buy it, don't touch it

PAST SITUATION (Type 3)  past perfet - would have

Here we are talking about an impossible situation because the condition took place in the past, and it never happened (if it happened then we don't talk about conditions but about facts)
 
      If you had married me, we would have been happy   (but you didn't)

SUBJUNCTIVE
 
In type 2  we use the simple past, but we are not talking about real actions in the past, only about hypotheses, that is why we call it "unreal past". It doesn't refer to the past but to the present, a present which is not real, which is not true. But in modern English, real past forms (indicative mode) and unreal past forms (subjunctive mode) are the same.
 
- If I knew it I would tell you (present)   vs   She told you because she knew it (past)
 
The only verb which still has one different form for the unreal past is the verb "to be". It uses WERE for all the forms in unreal past and the forms WAS or WERE for the real past, so I and He/She/It are different in indicative and subjunctive:
 
- If I were rich, I'd buy a big house in Paris
 
But this remaining distinction is also disappearing, and now it is frequent to hear:
 
- If I was German I could speak German perfectly
 
But there is still one situation when we never say "I was", and that is when giving advice with the expression "If I were you":
 
- If I were you, I wouldn't tell her 

Only for Spanish Speakers

  I M P O R T A N T !!!

Las condicionales inglesas se construyen y se usan igual que en español, así que no deben resultar difíciles. Sin embargo hay una diferencia, y ahí es donde los hablantes de español comenten frecuentemente errores: El tipo 1 de condicionales (present - will) en español frecuentemente se construye como (present - present) aunque estemos hablando del futuro, así que ten mucho cuidado con esas situaciones:
 
    Si va tu padre yo no voy -------------- If your father comes, I won't go
    Si quieres un güisqui te lo pongo ---- If you want a whisky, I'll pour you one
    Si no me lo cuentas me enfado ------ If you don't tell me, I'll get angry
 
Usar en inglés present-present en estas frases sería incorrecto!!!
 
También hay que tener cuidado con un problema del español: en ciertas zonas, en vez de usar "pasado + condicional" la gente usa "pasado + pasado" o "condicional + condicional":
 
- Si vendrías lo verías (País Vasco, etc.) = Si vinieras lo verías = If you came, you'd see it
- Si tuviera dinero me lo compraba ahora mismo (Castilla, etc.) = Si tuviera dinero me lo compraría ahora mismo = If I had money, I'd buy it right now.

UNLESS = if...not (a menos que)