Si dejo la leche fuera, se daña rápido.

In Spanish, an if clause with si + present indicative often expresses a general result or a typical consequence:

• Si dejo la leche fuera, se daña rápido. = Whenever/if I leave the milk out, it spoils quickly.

You can make it clearly future:

• Si dejo la leche fuera, se dañará rápido. (If I leave it out, it will spoil quickly.)

And you’d use past if you’re talking about a specific past situation:

• Si dejé la leche fuera, se dañó rápido. (If I left the milk out, it spoiled quickly.)

Because this sentence talks about a realistic, possible condition. Spanish uses the indicative with si for real conditions:

• Si + present indicative → real/general condition

Subjunctive with si appears in unlikely/hypothetical conditions:

• Si dejara/dejase la leche fuera, se dañaría. (If I were to leave the milk out, it would spoil.)
• Si hubiera dejado la leche fuera, se habría dañado. (If I had left it out, it would have spoiled.)

Dañarse means to get damaged / to spoil / to go bad (depending on context). With food like milk, se daña commonly means it spoils.

The se here makes the verb intransitive (no external doer is mentioned). It’s like saying:

• It gets spoiled / It spoils

Compare:

• Se daña la leche. = The milk spoils (gets spoiled).
• La leche se daña. = same meaning, different word order.
• Alguien daña la leche. = Someone damages the milk (sounds odd for milk unless it’s intentional contamination).

It’s closer to a middle/“involuntary change” use of se (often grouped with reflexive uses), not the classic passive with ser.

Spanish also has a true se-passive like:

• Se venden casas. = Houses are sold.

But se daña rápido is more like “it ends up spoiled quickly,” focusing on the result rather than on an agent.

Here fuera means out (not put away / not refrigerated). With milk, it usually implies left out of the fridge.

If you want to be extra clear:

• Si dejo la leche fuera del refrigerador/la nevera, se daña rápido.

Often yes, and many Latin American speakers may prefer afuera in casual speech:

• Si dejo la leche afuera, se daña rápido.

A rough nuance:

• fuera can feel a bit more “out (of its proper place)”
• afuera can sound more literally “outside,” but in context it also works as “left out”

Both are commonly understood.

In Spanish, it’s very common (and recommended) to put a comma between the si-clause and the result clause when the si-clause comes first:

• Si X, Y.

If you reverse the order, the comma usually disappears:

• Se daña rápido si dejo la leche fuera.

Spanish often uses the definite article (el/la) when talking about something in a general, everyday way, especially with common household items:

• dejar la leche fuera = leave the milk out

You can omit it in some contexts (especially in more general statements), but it may sound less natural here:

• Si dejo leche fuera… can sound like “if I leave some milk out” (more indefinite).

Here rápido works as an adverb in everyday Spanish: quickly. Using the adjective form as an adverb is very common:

• Habla rápido. = He speaks fast.
• Se daña rápido. = It spoils quickly.

Rápidamente is also correct, just a bit more formal or emphatic:

• Se daña rápidamente.

Yes—very common alternatives are:

• Se echa a perder. (It goes bad / it gets ruined.)
  Si dejo la leche fuera, se echa a perder rápido.
• Se pone mala. (It becomes bad.)
  Si dejo la leche fuera, se pone mala rápido.

Se daña is widely understood and common too.