CONDITIONAL SENTENCE

Conditional tenses are used to speculate about what could happen, what might have happened, and what we wish would happen. In English, most sentences using the conditional contain the word if. Many conditional forms in English are used in sentences that include verbs in one of the past tenses. This usage is referred to as "the unreal past" because we use a past tense but we are not actually referring to something that happened in the past. There are five main ways of constructing conditional sentences in English. In all cases, these sentences are made up of an if clause and a main clause. In many negative conditional sentences, there is an equivalent sentence construction using "unless" instead of "if".

Conditional sentence type	Usage	If clause verb tense	Main clause verb tense
Zero	General truths	Simple present	Simple present
Type 1	A possible condition and its probable result	Simple present	Simple future
Type 2	A hypothetical condition and its probable result	Simple past	Present conditional or Present continuous conditional
Type 3	An unreal past condition and its probable result in the past	Past perfect	Perfect conditional
Mixed type	An unreal past condition and its probable result in the present	Past perfect	Present contditional

The zero conditional

The zero conditional is used for when the time being referred to is now or always and the situation is real and possible. The zero conditional is often used to refer to general truths. The tense in both parts of the sentence is the simple present. In zero conditional sentences, the word "if" can usually be replaced by the word "when" without changing the meaning.

If clause	Main clause
If + simple present	simple present
If this thing happens	that thing happens.
If you heat ice	it melts.
If it rains	the grass gets wet.

Type 1 conditional

The type 1 conditional is used to refer to the present or future where the situation is real. The type 1 conditional refers to a possible condition and its probable result. In these sentences the if clause is in the simple present, and the main clause is in the simple future.

If clause	Main clause
If + simple present	simple future
If this thing happens	that thing will happen.
If you don't hurry	you will miss the train.
If it rains today	you will get wet.

Type 2 conditional

The type 2 conditional is used to refer to a time that is now or any time, and a situation that is unreal. These sentences are not based on fact. The type 2 conditional is used to refer to a hypothetical condition and its probable result. In type 2 conditional sentences, the if clause uses the simple past, and the main clause uses the present conditional.

If clause	Main clause
If + simple past	present conditional or present continuous conditional
If this thing happened	that thing would happen. (but I'm not sure this thing will happen) OR that thing would be happening.
If you went to bed earlier	you would not be so tired.
If it rained	you would get wet.
If I spoke Italian	I would be working in Italy.

Type 3 conditional

The type 3 conditional is used to refer to a time that is in the past, and a situation that is contrary to reality. The facts they are based on are the opposite of what is expressed. The type 3 conditional is used to refer to an unreal past condition and its probable past result. In type 3 conditional sentences, the if clause uses the past perfect, and the main clause uses the perfect conditional.

If clause	Main clause
If + past perfect	perfect conditional or perfect continuous conditional
If this thing had happened	that thing would have happened. (but neither of those things really happened) OR that thing would have been happening.
If you had studied harder	you would have passed the exam.
If it had rained	you would have gotten wet.
If I had accepted that promotion	I would have been working in Milan.

Source:

http://www.ef.com/english-resources/english-grammar/conditional/

https://www.ego4u.com/en/cram-up/grammar/conditional-sentences