A conditional sentence consists of two clauses:  1) The if-clause which is called the PROTASIS (that which is set out beforehand, the condition) and 2) The result clause which is called the APODOSIS (that which is given back, the response, the conclusion, the result, the consequence).

Here's an example:

Usually the PROTASIS comes before the APODOSIS (as in the example above), but not always (see Hebrews 3:6 and 6:3 where the APODOSIS comes first).

In Greek, there are four classes of conditional sentences:

"The first class condition indicates the assumption of truth for the sake of argument. The normal idea, then, is if--and let us assume that this is true for the sake of argument--then...." (Daniel Wallace, Greek Grammar Beyond the Basics, p. 690).

General Example:

"If this is true (and I'm assuming it is), then this will happen."

Specific Example:

"If you believe on Christ (and I'm assuming you really do), then you are saved"

Note:  The person in reality may not believe on Christ, but the speaker assumes that he is a true believer.  His assumption may be incorrect.

Form:

The PROTASIS has ει (if) with the indicative mood.  For an explanation of the indicative mood, see Chapter 8.

In the first class conditional sentence, the speaker or writer assumes the condition to be true.  Whether it is actually true is another story!  The condition (PROTASIS) is merely assumed to be true for the sake of argument.  He may know that the condition is false, but he assumes that it is true for the sake of argument.  Whether the PROTASIS is really true must be determined by the context and by common sense.  Don't be deceived by the fact that the indicative mood (the mood of factual reality) is used in the PROTASIS.  Most LIES are told in the indicative mood!  Lies are stated as facts because the LIAR wants you to believe that his statement is true (compare Genesis 3:4).  Also, beware lest you translate "if" as "since" or "in view of the fact."  Even in Colossians 3:1 (where "since" seems to make good sense) Paul is assuming something to be true, although it may not be true.  Paul assumed that the Colossians were true believers in Christ and thus that they were "risen with Christ."  But it is very possible that there was an unsaved person in the Colossian church who was among those who first heard this letter read. Thus we could paraphrase the verse as follows:

"If you have been risen with Christ (and I'm assuming you have been, because I'm assuming that I'm writing to saved people), then seek those things which are above" (Col. 3:1).

Matthew 12:27 is an example of a first class conditional sentence which must not be translated as "since."  Jesus said, "If I cast out demons by Beelzebul, by whom do your sons cast them out?"  Why would it be totally wrong to translate "if" as "since" in this verse? 

To prove an argument, a person skilled in logic will often take a statement which he knows to be false (but which his opponents believe is true) and he will assume that it is true in order to show the absurd and illogical conclusions that follow.  Thus we have Paul's masterful arguments in:

1 Corinthians 15:13,14,16,17,19 (compare verse 20)

Romans 4:2 (compare verse 3)

Galatians 5:11

Please note that in the above examples it would be incorrect to translate "if" as "since."

Notice also the art of argumentation as exemplified by our Lord:

John 10:37-38

John 15:20

John 18:23

Matthew 12:26-28

It is possible for people to think something is false and then assume it to be true for the sake of argument, but in reality that which they assumed to be true (but thought to be false) was really true!  If this last sentence seems confusing, see the example in Luke 23:35,37.

Here are some other examples of First Class Conditional Sentences:

"The second class condition indicates the assumption of an untruth (for the sake of argument).  For this reason it is appropriately called the "contrary to fact" condition (or the unreal condition). It might be better to call it presumed contrary to fact, however, since sometimes it presents a condition that is true, even though the speaker assumes it to be untrue (e.g., Luke 7:39).  --Wallace, p. 694.

General Example:

"If this would have been, then that would have followed."

Specific Example:

"If you had believed on Christ (but you didn't), then you would have been saved."

Other examples in English:

If I were you, I would not go.
If you were older, you would understand.
If this were Sunday, I would be at church.

Form: 

The PROTASIS has εἰ (if) with the indicative mood and the APODOSIS usually has άν with the indicative, although sometimes άν is omitted.

In the second class conditional sentence, the speaker assumes the condition to be untrue.  Actually the condition may be true, but the speaker believes or assumes it to be untrue:   See Luke 7:39.

The PROTASIS is assumed to be untrue or contrary to fact.  The APODOSIS tells you what would have happened if conditions had been different (see John 11:21,32).

Here are some other examples of second class conditional sentences:

"The third class condition often presents the condition as uncertain of fulfillment, but still likely.  There are, however, many exceptions to this...The third class condition encompasses a broad range of potentialities in Koine Greek. It depicts what is likely to occur in the future, what could possibly occur, or even what is only hypothetical and will not occur" (Wallace, p. 696).

General Example:

"If at any future time this condition is met, then this will follow."

Specific Example:

"If you should believe on Christ (and I hope you will), then you will be saved."

Form: 

The PROTASIS has εαν (ει + αν) with the subjunctive mood.

The third class conditional sentence states what results will happen if at any time in the future the condition is met.  The writer or speaker knows that at the present time the condition has not been fulfilled, but it is possible and even probable that the condition will be met at some time in the future.  There is hope that the condition will be fulfilled, but on the other hand, there is some uncertainty or doubt in the writer's mind as to whether the condition will be met.  It may be fulfilled, or it may not be fulfilled.  Consider 1 John 1:9.  If at any time in the future (the sooner the better) the believer confesses his sins, then God will forgive and cleanse!  Hopefully, the believer will confess his sins, but there is also the possibility that he will fail to do this. But the moment he confesses, God will forgive and cleanse.  If the believer fulfills the PROTASIS, then God will fulfill the APODOSIS! If I do my part, God will do His part!

Here are some examples of third class conditional sentences:

Compare John 13:17 where both a first and a third class condition are found.

"The fourth class condition indicates a possible condition in the future, usually a remote possibility...There are no complete fourth class conditions in the NT" (Wallace, p. 699).

Form: 

The PROTASIS has ει with the optative mood.

The fourth class condition is even less probable than the third class condition.  Whereas the fulfillment of a third class condition is PROBABLE, the fulfillment of a fourth class condition is merely POSSIBLE. There are no complete examples of a fourth class conditional sentence in the New Testament. For a partial example, see 1 Peter 3:14 ("But even if ye should suffer for righteousness sake, happy are ye").

The optative mood rarely occurs in the New Testament.