This document describes the four types of conditional sentences in English: 1) Type 0 or "Zero Conditional" uses the simple present tense and describes general truths or laws. 2) Type 1 or "First Conditional" uses the present tense in the if-clause and will or may in the main clause to talk about probable future events. 3) Type 2 or "Second Conditional" uses the past tense in both clauses to talk about hypothetical or imaginary situations in the present or future. 4) Type 3 or "Third Conditional" uses the past perfect tense in both clauses to talk about imaginary situations in the past.

1. http://www.xtec.es/~ogodoy/sac/rephrasing/conditionals.htm)
Types 0, 1, 2 & 3
MAIN
IF-CLAUSE MEANING
CLAUSE
General conditions
General laws; things
Simple present Simple present
("Zero" Conditional) that are always true
WILL + V
Possible conditions
(also CAN, MAY, Probable future
Simple present
(1st Conditional) MUST & result
imperatives)
Hypothetical WOULD + V
Imaginary situation
conditions
Simple past (also COULD / in the present or
(2nd Conmditional) MIGHT) future
Impossible WOULD HAVE + pp
conditions Imaginary situation
Past Perfect
(also COULD / in the past
(3rd Conditional) MIGHT)
UNLESS can be used with the meaning "IF ... NOT" in general and possible
conditions only.
Other structures
 As long as, providing (or provided that) and on condition: these
expressions mean only if and can be used with the same verb forms as
conditional sentences:
In Spain you can drive a car as long as you are 18
 Inversion of subject and verb: this structure is very common in formal
English in conditional sentences with should and the past perfect. When
inversion is used, we omit if:
Had I known about the transport strike, I would have taken my car.
Should you meet Sheila, tell her the meeting has been postponed.