The conditional of real situations
The periodo ipotetico della realtà describes a condition that can actually come true, and the consequence that follows when it does. It may rain, and if it rains, I stay home. English says If it rains, I'll stay home: an ordinary present after if. Italian does the same, with the indicativo presente after se.
What goes in the main clause
After the se clause the main clause has three usual options.
Present — the default
The present covers habits and rules. In everyday speech it also stands for the near future, where English says I'll: Se ho tempo, vengo.
Se piove, resto a casa.
If it rains, I stay home.
Se mangio troppo la sera, non dormo bene.
If I eat too much in the evening, I don't sleep well.
Se ho tempo, vengo.
If I have time, I'll come.
Future — an event still ahead
For something that has not happened yet, the main clause can move into the futuro. The se clause stays in the present, as in English If I have time, I'll come.
Se studi abbastanza, passerai l'esame.
If you study enough, you'll pass the exam.
Se partiamo adesso, arriveremo in orario.
If we leave now, we'll arrive on time.
Here Italian allows something English does not: the futuro may stand in the se clause too, Se pioverà, resteremo a casa, although English never says if it will rain. It sounds more emphatic and is less common in everyday speech.
Imperative — advice and instructions
Instead of a consequence, the main clause can give an instruction, typical for advice. English does this too: If it rains, take an umbrella.
Se piove, prendi l'ombrello.
If it rains, take the umbrella.
Se fai tardi, avvisami.
If you're running late, let me know.
Word order and the comma
The se clause can stand first or last, and the meaning does not change. In front, a comma separates the two halves. At the end, there is no comma. English punctuates the same way.
Se non ti va, non veniamo.
If you don't feel like it, we won't come.
Non veniamo se non ti va.
We won't come if you don't feel like it.
se or quando?
You already make this distinction in English, and it maps one to one. Se is if: you do not know whether it will happen. Quando is when: it is going to happen, only the timing is open.
Open against certain
se — if
The condition is open. Test: if English if fits, Italian takes se.
Se piove, non usciamo.
If it rains, we're not going out.
Se arriva Luca, avvisami.
If Luca turns up, let me know.
quando — when
The event is settled. Test: if English when fits, Italian takes quando.
Quando arriva l'estate, andiamo al mare.
When summer comes, we go to the seaside.
Quando finisco di lavorare, ti chiamo.
When I finish work, I'll call you.
