The theory of conditionals in Vietnamese grammar revolves around the use of conditional sentences. These sentences are used to express hypothetical or unreal situations and their possible outcomes. There are three main types of conditional sentences in Vietnamese: the zero conditional, the first conditional, and the second conditional.

The zero conditional is used to express general truths or facts. It consists of two clauses, each containing a present tense verb. For example, “Nếu bạn học chăm chỉ, bạn sẽ đạt được thành công” (If you study hard, you will achieve success).

The first conditional is used to talk about possible future actions and their probable outcomes. It consists of two clauses, the first containing the present simple tense and the second containing the future simple tense. For example, “Nếu tôi có thời gian, tôi sẽ đến thăm bạn” (If I have time, I will come visit you).

The second conditional is used to talk about unreal or unlikely situations in the present or future. It consists of two clauses, the first containing the past simple tense and the second containing the conditional tense. For example, “Nếu tôi biết, tôi sẽ kể cho bạn biết” (If I knew, I would tell you).