The quick answer

Use the first conditional for a real, likely situation in the future and its probable result: if + present simple in the condition, will + base verb in the result. If it rains, we will stay home. The if-clause never takes will — that is the single biggest mistake at this level.

The rule

The first conditional describes something that can really happen, not something imaginary. The condition clause states the possible future event using the present simple, and the result clause states what will probably happen using will + base verb. The two clauses can swap order: If you study hard, you will pass means the same as You will pass if you study hard. A comma follows the if-clause only when it comes first.

Form

Examples

• If you study hard, you will pass the exam.
• She will call you if she has time tonight.
• If they do not hurry, they will miss the train.
• We will go to the beach if the weather is good.
• If he does not apologise, she will not forgive him.

Common mistakes

• ❌ If it will rain, we will stay home. → ✅ If it rains, we will stay home. The if-clause always uses the present simple, never will.
• ❌ If you study hard, you pass the exam. → ✅ If you study hard, you will pass the exam. The result clause needs will, or the sentence sounds like a general fact, not a prediction.
• ❌ If she will not call, I am angry. → ✅ If she does not call, I will be angry. Keep will only in the result clause, and use will be, not am, for a future feeling.
• ❌ If it rains, we would stay home. → ✅ If it rains, we will stay home. Would belongs to the second conditional, for unreal or unlikely situations, not real future ones.