The conditional in English

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Type 1. Open if clause - the open conditional statement :

This usually refers to a future event which is conditional on another future event.
The verb of the main clause is in the future tense with "will" (or sometimes another modal) and the verb of the conditional clause is in the simple present tense. - examples 1 & 2.
 
But when the open conditional statement describes one perrmanent state of reality or circumstance which is dependent on another, in this case, both verbs are in the present tense - examples 3 - 5. (In such cases, if can be replaced by when.)

In an open conditional statement, if is sometimes replaced by when: but there is a difference. Using "if" implies that the condition really is open and may not be fulfilled, using "when" implies that the condition will be fulfilled, that the event will really take place.

Type 2 if clauses - the open hypothetical conditional statement:

This refers to a possible future situation which depends on on another possible future situation. The verb of the main clause uses the present conditional tense (would + infinitive, or could +infinitive);
The verb of the conditional clause normally uses the present subjunctive or preterite (these two tenses are identical except with to be). Occasionally, the conditional aspect of the statement can be emphasised by using the form were + to + infinitive.
Note also this common expression (which uses the open hypothetical form, though it is clearly quite impossible!)

This form is also used in cases of reported speech.

Type 3 if clauses - the unfulfilled hypothesis 

This refers to a situation which an event might have taken place, but did not, because a condition was not fulfilled.
The verb of the main  clause goes in the past conditional (would have + past participle).
The verb of the conditional clause goes in the past perfect (had + past participle).
 Examples:
 

Note: using " unless"

"Unless" means the same as "if ... not", and has a negative value. It is frequently (but not only) used in conditional statements where the verb of the main clause is also  in the negative.

4a  Omission of "if", with inversion.

Sometimes, hypothetical conditional statements or unfulfilled hypothetical statements can be expressed omitting the word if.  
When this happens the subject follows the auxiliary verb in the conditional clause
Examples:

4b  Whether

Whether can sometimes replace if, when there is a choice of conditions. It implies

1. a stated choice between two options (example 1, using whether... or) ,
2. or a choice between an affirmative option and a stated negative option  (examples 2) ,
3. or a choice between an affirmative option and an implied or stated  negative option in an indirect question (example 3).

See correlating coordinators

In examples 1 and 2a, it would not be usual to use if in place of whether.
In example 2b, whether must be used. It is very unusual to say or write  "if or not"
In example 3,  there is a free choice between whether and if. Both can be used, though whether is preferred when followed by if not.