Identical Zero and 1st Conditional

The zero conditional, when with a modal, looks identical to the 1st conditional structure.

Structural comparison

Purpose	Formula	Example
Zero — general facts, habits, rules	If + Present + Present	If I win, you come.
	If + Present + Modal	If I win, you can come.
First — specific future outcome	If + Present + Future	If I win, you will come.
	If + Present + Imperative	If I win, come.
	If + Present + Modal	If I win, you can come.

When modal verbs (must, can, should, may, have to) are introduced, they replace standard verb tenses and blur the line between zero and first conditionals.

Both zero and first conditionals can be made with Present + Modal: If I win, you can come.

• If this sentence refers to a situation that always happens, it is the zero conditional, meaning: Every time I win, you can always come.
• If it refers to a specific future situation, it is the first conditional, meaning: If I win (tomorrow), you can come (at that time tomorrow).

In order to identify which is the case, look at the context.

Further examples

1. Can (ability / permission)

• Zero conditional (general rule): If you have a library card, you can borrow books.
• First conditional (specific future): If you pass your test tomorrow, you can borrow my car.

2. Should (advice / recommendation)

• Zero conditional (general advice): If it rains, you should wear a coat.
• First conditional (future advice): If it rains this afternoon, you should take an umbrella.

3. Have to (obligation)

• Zero conditional (general duty): If the alarm goes off, everyone has to evacuate.
• First conditional (future duty): If the alarm goes off during our meeting later, we have to leave immediately.