Ayer olvidé remojar las lentejas, así que hoy tendré que preparar otra cosa.

Because olvidé is the preterite form of olvidar for yo: I forgot.

The sentence starts with Ayer, which points to a completed action in the past, so the preterite is the natural choice:

• Ayer olvidé... = Yesterday I forgot...

If you used the imperfect, olvidaba, it would sound more like an ongoing or repeated background action, which does not fit as well here.

Spanish often drops subject pronouns when the verb ending already makes the subject clear.

Here, olvidé clearly means I forgot, and tendré clearly means I will have, so yo is unnecessary.

You could say Yo olvidé... or Yo tendré..., but that would usually add emphasis or contrast.

Both are possible.

• olvidar + infinitive = to forget to do something
• olvidarse de + infinitive / noun = also to forget, very common in everyday speech

So these are both natural:

• Olvidé remojar las lentejas
• Me olvidé de remojar las lentejas

In Spain, you may also hear:

• Se me olvidó remojar las lentejas

That version often sounds a bit more accidental or less direct, like It slipped my mind.

Remojar means to soak.

In this sentence, it means soaking the lentils in water before cooking them.

In Spain, a very common alternative is:

• poner en remojo

So you could also say:

• Ayer olvidé poner las lentejas en remojo...

That may sound even more idiomatic to many speakers in Spain.

Spanish often uses the definite article with foods, ingredients, and things that are understood in context.

Here, las lentejas means the lentils—the lentils the speaker planned to cook.

English often leaves the article out in similar situations, but Spanish commonly keeps it.

Así que means so, therefore, or as a result.

It connects the first idea to its consequence:

• I forgot to soak the lentils yesterday, so today I’ll have to make something else.

It is a very common and natural connector in Spanish.

They can all relate ideas, but they are not used in exactly the same way.

• así que = so / therefore, very natural for linking cause and result
• entonces = then / so, often a bit looser or more conversational
• por eso = for that reason / that’s why

In this sentence, así que is a smooth way to say so.

For example:

• Ayer olvidé remojar las lentejas, así que hoy...
• Ayer olvidé remojar las lentejas; por eso hoy...

Both work, but así que sounds especially natural here.

Tendré que means I will have to.

It is built like this:

• tendré = future of tener for yo
• que
• infinitive verb

So:

• tendré que preparar = I will have to prepare

Note that tener has an irregular future stem:

• tener → tendr-
• tendré, tendrás, tendrá...

Because after tener que, Spanish uses the infinitive.

So the structure is:

• tener que + infinitive

Examples:

• Tengo que estudiar = I have to study
• Tuve que salir = I had to leave
• Tendré que preparar = I will have to prepare

So preparar is correct here.

Otra cosa means something else or another thing.

Here it means the speaker will have to make a different dish or meal instead of lentils.

• otra = other / another
• cosa = thing

Because cosa is feminine singular, the adjective is otra, not otro.

Yes. Both are possible, but the nuance is slightly different.

• preparar otra cosa = prepare something else
• hacer otra cosa = make/do something else

With food, both can work. Preparar sounds a bit more specifically about cooking or meal preparation, while hacer is broader and very common in everyday speech.

Spanish word order is flexible, and time expressions are often placed near the beginning for emphasis.

Here, the sentence highlights a contrast:

• Ayer = what happened yesterday
• hoy = the consequence today

That makes the timeline very clear.

A different order is also possible:

• Olvidé remojar las lentejas ayer, así que hoy tendré que preparar otra cosa.

But the original version sounds very natural.

Yes.

• tendré que preparar otra cosa
• voy a tener que preparar otra cosa

Both mean I’ll have to prepare something else.

A small difference:

• tendré que = simple future, a bit more neutral or compact
• voy a tener que = very common in spoken Spanish, often feels a bit more immediate

In everyday conversation in Spain, voy a tener que is extremely common.

The accent marks show stress and sometimes help distinguish forms.

• olvidé: stress on the last syllable, and it marks the yo preterite
• así: stress on the last syllable
• tendré: stress on the last syllable

They are important both for pronunciation and for correct spelling.

The sentence is completely correct and natural.

That said, in Spain you may also hear more everyday versions such as:

• Ayer se me olvidó poner las lentejas en remojo, así que hoy tendré que hacer otra cosa.

Why?

• se me olvidó is very common in conversation
• poner en remojo is a very common way to say soak
• hacer otra cosa is also very everyday

So the original sentence is good Spanish, and there are also slightly more colloquial alternatives.