There are a number of structures in English
that are called the conditionals which are used to talk
about possible or imaginary situations.
A "Condition" is a "situation or circumstance".
For example: If a certain condition is true,
then a particular result happens.
Conditions are situations that must happen to change
The structure of the conditionals is straightforward.
There are two basic possibilities in terms of order in the sentence:
A conditional sentence has two parts:
The if clause, which tells us about the condition.
The result clause, which tells us about the situation
These clauses can be in either order.
There are four well-known conditional forms as follows:
When or 'If' + present verb ,
present verb (Without WILL) .
Note: When rather than if
can give a greater feeling of certainty.
If + present verb , will + present verb.
Note: There are other possible modals besides
will and other words apart from If.
For example: Unless
If ... + past simple, .... +
would (will in the past) + bare infinitive ..
If ... had been (past perfect) ...,
.... would have been (will in the past + present perfect) ....
Mixed conditionals change tense in one clause so that
the structure does not match the zero, first,
second and third conditionals.
Sometimes these sentences are correct if the meaning is clear
and there is a good reason for the tense change.
The most common mixed conditional is a condition in the past with a result in the present.
It starts like a 3rd conditional but ends with would + infinitive.
If I had been a singer, I would be rich! The If clause is in the 3rd conditional,
so it's an unreal past, and 'would be' (2nd conditional) is an unreal present.
Unreal past condition and unreal present result.
If he had been up the Eiffel Tower, he would know it's in Paris. He doesn't know the Eiffel Tower is in Paris,
so he hasn't been up the Eiffel Tower.
Unreal past condition, unreal present result.
If she had caught the train, she would be here by now. Unreal past condition and unreal present result.
If I had done the homework, I would be ready. Unreal past condition and unreal present result.
Other mixed conditionals
Although the mixed conditional form past perfect, + would + bare infinitive
shown here is the most common. There are many other possible mixed conditionals.
If a sentence makes clear sense, in terms of the tense meaning and it is different to
all the standard conditional forms,
then it may stand as a correct sentence.
If she has got on the plane, she'll be in New York in the morning.
If she had got (3rd conditional) on the plane,
she'd be (2nd conditional) in New York in the morning.