In this first conditional sentence, the simple present tense is used in the if-clause, and the tense in the main clause is the simple future tense.

'If' Clause

Main Clause

If + simple present tense

Simple future tense

If you leave now

You'll arrive home early.

 

This conditional shows the outcome (you’ll arrive home early) if the condition is fulfilled (if you leave now). We use this conditional when the situation in the if-clause appears likely and there is a possibility of it happening.

 

We can turn the two clauses the other way round so that their positions are reversed as follow:

Simple future tense

if + simple present tense

You'll arrive home early

If you leave now.

 
Whether the if-clause or the main clause comes earlier, both conditional sentences are acceptable.

 

More examples:

The first conditional sets up a present condition and a probable future outcome.

  • If you help me with this, I will buy you a drink. (If you fulfill the condition in the if-clause by helping me, the second clause will result, that is you get a drink from me.
  • If I see him tomorrow, I will tell him the good news.
  • If the weather remains fine, we will go to the beach.
  • If you continue to feel better, you will be out of the hospital soon.

We may use going to in the first conditional to refer to a future event.

  • If she passes her driving test, we are going to celebrate at a good restaurant.
  • If you don’t hurry, we are going to be blacklisted for being late.