Si se te olvida descongelar el pescado, será mejor cambiar el menú de la cena.

Because Spanish often uses olvidarse in a pattern similar to gustar:

• Se te olvida X = you forget X
• Literally, it is closer to X slips your mind

In this structure:

• se is part of the verb pattern
• te means to you
• the thing being forgotten is the grammatical subject

So se te olvida descongelar el pescado literally works like:

• To you, forgetting to defrost the fish happens
• more naturally: you forget to defrost the fish

You can say si olvidas descongelar el pescado, and it is correct. But si se te olvida... often sounds more natural when talking about forgetting something accidentally or absent-mindedly.

Because in se te olvida, the verb agrees with the thing forgotten, not with you.

Here, the subject is the infinitive phrase:

• descongelar el pescado

An infinitive phrase counts as singular, so the verb is singular:

• se te olvida descongelar el pescado

Compare:

• Se te olvida descongelar el pescado = You forget to defrost the fish
• Se te olvidan las llaves = You forget the keys

In the second example, las llaves is plural, so the verb becomes olvidan.

In se te olvida:

• te = to you
• se is part of this common pronominal construction

A very useful way to understand it is:

• Se me olvida = I forget
• Se te olvida = you forget
• Se le olvida = he/she/you formal forgets
• Se nos olvida = we forget
• Se os olvida = you all forget, in Spain
• Se les olvida = they/you all formal forget

So the person is expressed through the indirect object pronoun, not by a normal subject pronoun.

Because after olvidar or olvidarse when you mean forget to do something, Spanish uses an infinitive:

• olvidar + infinitive
• olvidarse de + noun
• olvidársele + infinitive / noun

So:

• olvidar descongelar el pescado = to forget to defrost the fish
• se te olvida descongelar el pescado = you forget to defrost the fish

This is the normal way to say forget to do something.

Yes, that is also possible.

There are a few related patterns:

• olvidar algo = to forget something
• olvidarse de algo = to forget something
• olvidar + infinitive = to forget to do something
• olvidarse de + infinitive = to forget to do something
• olvidársele algo / un infinitivo = for something to slip one’s mind

So these are all possible:

• Si olvidas descongelar el pescado...
• Si te olvidas de descongelar el pescado...
• Si se te olvida descongelar el pescado...

The sentence you were given sounds especially natural for an accidental lapse of memory.

Because this is a real, possible condition.

Spanish commonly uses:

• si + present indicative, then future / imperative / present

So:

• Si se te olvida descongelar el pescado, será mejor cambiar el menú de la cena.

This means something like:

• If you forget to defrost the fish, it’ll be best to change the dinner menu

This is different from hypothetical patterns such as:

• Si se te olvidara..., sería mejor... = If you were to forget..., it would be better...

So the original sentence is talking about a realistic possibility, not a remote or imaginary one.

Yes, será is the future of ser, and literally será mejor means it will be better.

But in context, it often sounds like:

• it’d be better
• it will be best
• we’d better

So here:

• será mejor cambiar el menú de la cena

means something like:

• it’ll be better to change the dinner menu
• we’d better change the dinner menu

It is a very natural way to make a practical recommendation.

Because Spanish often uses an impersonal structure:

• será mejor + infinitive

This means:

• it will be better to...

So:

• será mejor cambiar el menú de la cena
  = it will be better to change the dinner menu

Spanish does not need to say who exactly is doing the action if it is clear from context. In English, we might naturally translate it as we’d better change the dinner menu, even though we is not explicitly stated in Spanish.

Spanish often uses the definite article with food items and many nouns where English would not.

Here, el pescado means:

• the fish

It probably refers to the fish that was going to be prepared for dinner.

Spanish generally uses articles more often than English, especially with specific things already understood in the situation.

Compare:

• Voy a preparar el pescado = I’m going to prepare the fish
• Compra pan = Buy bread
• El pan está en la mesa = The bread is on the table

It modifies el menú:

• el menú de la cena = the dinner menu

So the idea is not the fish of the dinner, but rather the menu for dinner.

Structure:

• cambiar = to change
• el menú = the menu
• de la cena = for dinner / of the dinner

In natural English, you would usually say the dinner menu.

Yes. te is the informal singular you.

Here are the main changes:

• Si se te olvida... = if you forget... informal singular
• Si se le olvida... = if you forget... formal singular
• Si se os olvida... = if you all forget... informal plural in Spain
• Si se les olvida... = if you all forget... formal plural, or common in Latin America

Examples:

• Si se le olvida descongelar el pescado, será mejor cambiar el menú de la cena.
• Si se os olvida descongelar el pescado, será mejor cambiar el menú de la cena.

No, not in this sentence.

After si for a real condition, Spanish normally uses the indicative, not the present subjunctive:

• Si se te olvida... ✔
• Si se te olvide... ✘

The subjunctive is not used after si in standard conditional sentences of this kind.

Useful rule:

• si + present indicative for real possibilities
• si + imperfect subjunctive for hypothetical situations

So:

• Si se te olvida, será mejor... = If you forget, it’ll be better...
• Si se te olvidara, sería mejor... = If you forgot / were to forget, it would be better...

In Spain, descongelar is the normal everyday verb for to defrost or to thaw food.

• descongelar el pescado = to defrost the fish

Deshelar exists, but it is much less common in everyday kitchen contexts and is more associated with de-icing or removing ice in a more literal sense. For food, descongelar is the natural choice.

Because the sentence begins with a conditional clause:

• Si se te olvida descongelar el pescado

and then follows with the main clause:

• será mejor cambiar el menú de la cena

In Spanish, when a si clause comes first, it is normal to separate it with a comma.

So the structure is:

• If X happens, Y

That is exactly what the comma is marking.