Conditionals theory in English grammar refers to the study of conditional sentences, which are used to express different possibilities or hypothetical situations. These sentences are formed by combining a main clause and a subordinate clause, and their structure depends on the type of condition being expressed.

There are four basic types of conditional sentences: zero conditional, first conditional, second conditional, and third conditional. The zero conditional is used to express general truths or facts, while the first conditional is used to talk about future possibilities. The second conditional is used to express hypothetical or unreal situations in the present or future, while the third conditional is used to talk about past unreal situations.

In addition to these four types, there are also mixed conditionals, which combine elements from different types of conditional sentences. Understanding the different types of conditionals and when to use them is crucial for effective communication in English. By mastering this theory, learners can express their thoughts accurately and convey different degrees of certainty or probability.