Take a look at the following examples:
- If you push this button, the machine starts
- If we are late, Ann will be angry
- If I won the lottery, I would buy a new house
The first sentence refers to general truths. These are facts which always have an obvious consequence. In the second sentence, there is a high probability that something will happen. While in the third sentence you can see the phenomenon of unreal present. Whatever the condition is, it is not probable and the situation is not truth.
So first conditionals refer to general facts and truths. Second conditionals refer to probability in the future while third conditionals refer to unreal events in the present or future.
Zero conditional: if + Simple Present + Simple Present
First Conditional: if + Simple Present + will / might + verb 1
Second Conditional: if + Simple Past + would / could + verb 1
- The clauses can be interchangeable
Now let us take a look at the word unless. Unless can replace if when we talk about what we would do if something does not happen. We use it in First Conditional and it is not followed by negative sentence:
I won’t call the police unless you pay for my broken window
As you see, if an action happens, the person will not do what he is planning to do.