Conditional Structures

Conditionals are sentences with two clauses: an ‘if' clause and a main clause.
Four basic conditional structures are used in English.

• Zero Conditional
• First Conditional
• Second Conditional
• Third Conditional

Zero Conditional	What is always true or usually happens, or what is advised in this situation.
If	+Present Simple	+ Present Simple
If	you have a ticket	you go straight in.
If	you stand in the rain	you get wet.
If	you see an accident	call an ambulance.
The 'if'  in the above sentences can usually be replaced by 'when' without changing the meaning.
First Conditional	Something that will possibly happen in the future.
If	+Present Simple	+Will + Infinitive
If	I see an accident	I will call a doctor.
If	it rains tomorrow	the picnic will be cancelled.
If	you make an apple tart	Dad will eat it.
Note : Sometimes we use can or may instead of will. For example :  If you finish your vegetables, you can have a dessert.
Second Conditional	Imaginary events that are probably not going to happen.
If	+Past Simple	+Would + Infinitive
If	I saw an accident	I would call a doctor.
If	he had more money	he would buy a new car.
If	she worked harder	her results would improve.
Note : Sometimes we use could or might instead of would. For example : If we invited the author, he might come.   If we ordered a larger quantity, we could ask for a reduction.
Third Conditional	Something that did not happen, and the reaction or result you imagine  if it really had happened.
If	+Past Perfect	+Would + have + Past Participle
If	I had seen the accident	I would have called a doctor.
If	he had read the instructions	he would have assembled it faster.
If	she had stayed in Paris	she would have improved her French .