Zero conditional sentences refer to a general truth that is always considered to be true or unchallenged. Both the if-clause and the main clause use the habitual tense. The habitual present tense is a simple, unmarked form of the English verb.
(a) “If you heat ice, it melts.”
Zero conditional sentences do not have a specified event time since the result would always be the same regardless of when the described situation occurs. For example, (a) refers to a scientific fact that ice melts when it is heated. Thus, no matter when the ice is heated, it would always melt. In this case, it is suitable for the habitual tense to be used in both clauses since it can describe an event or state that always occurs or timeless events with an unspecified event time.
First conditional sentences refer to a real possible situation and its possible result. The present simple tense is used in the if-clause and the modal auxiliary “will” is used in the main clause.
(b) “If you get up late, you will miss the bus.”
Since the if-clause refers to possible events in the future, there is a possibility that the described event would not happen in the future. In example (b), it is still possible that the hearer will get up early. Therefore, instead of directly using the future tense, it is more suitable to use the [if + present simple] structure in the if-clause in which “if” shows the possibility, and the bare verb put after the conjunction “if” refers to future situations.
The main clause states the result of the possible event described in the if-clause. For example in (b), “miss the bus” is the result of the possible event “get up late”. Thus, it would be suitable to use the future tense marker “will” since the described result will definitely occur if the possible event happens.
Second conditional sentences refer to situations that are unreal or very unlikely to happen. The past simple tense is used in the if-clause and the modal auxiliary “would” is mainly used in the main clause.
(c) “If I were you, I would tell him the truth.”
Since the if-clause refers to the unreal or unlikely events, it would be suitable to use the past simple tense as the unreal past. The past tense can be used as the unreal past to distance people from what is being written or spoken, creating a hypothetical and imaginary sense of the described event. In example (c), “were” is used as the unreal past to mark the unrealistic and imaginary event of “I” becoming “you”, which can never happen.
The main clause states the probable result of the hypothetical situation in the if-clause. In (c), “I tell him the truth” is the possible result if “I” became “you”, which is just an imagination. Thus, the result is very unlikely to happen in the future. In this case, it is suitable to use the modal auxiliary “would” since it can be used to express irrealis modality. Irrealis modality is used to make non-factual or non-actual statements. The main clause is an irrealis statement since the described probable results refer to unlikely future possibilities and are not based on actual situations. When “would” is used to refer to the possibility of the speaker’s intention, it has a future-projecting sense and does not refer to actual past events. Thus, it can be used to express irrealis modality, making it suitable for the main clause.