1 conditional

If clause	Main clause
If + Present tense	will / can / may / must + verb
If it rains today,	I'll stay at home

• If it rains today, I'll stay at home.
  Si llueve hoy, me quedaré en casa.
• If he is busy now, I will come back tomorrow.
  Si está ocupado ahora, regresaré mañana.
• If I have time, I'll visit my parents this afternoon.
  Si tengo tiempo, visitaré a mis padres esta tarde.
• If it is warm tomorrow, we'll go to the beach.
  Si está caluroso mañana, iremos a la playa.
• If it is cold, you must wear warm clothes.
  Si está frío, debes usar ropa abrigada.
• If he doesn't do his homework, he can not go to the party.
  Si él no hace su tarea, no puede ir a la fiesta.
• If she doesn't call you, you can call her.
  Si ella no te llama, tú puedes llamarla.
• If you work hard, you may become a millonaire someday.
  Si trabajas duro, puede que te conviertas en un millonario algún día.
• If you spend more than you earn, you'll become a poor man.
  Si gastas más de lo que ganas, te convertirás en un hombre pobre.
• If they don't invite you, you must not go.
  Si ellos no te invitan, no debes ir.
• If we don't hurry, we'll miss our bus.
  Si no nos apuramos, perderemos nuestro autobus.
• If you pay now, you'll get a discount.
  Si pagas ahora, obtendrás un descuento.
• If they don't want to go out, they can stay home.
  Si no quieren salir, se pueden quedar en casa.
• If you drink too much, you'll get drunk.
  Si bebes demasiado, te emborracharás.
• If you feel sick, you must stay in bed.
  Si te sientes enfermo, debes quedarte en cama.
• If they don't come here, we'll have to go there.
  Si ellos no vienen aquí, tendremos que ir allí.

Note! We never use will, or won't in the if part of the sentence.

For example:

• If I have time today, I will phone my friend. = I will phone my friend if I have time today. (It is possible that I will have time - therefore the condition is real).
• If I go to England, I will buy some Cheddar cheese. = I will buy some Cheddar cheese if I go to England. (It is possible that I will go to England - therefore the condition is real).