Learn Hardcore Spanish (Spain): Lo importante es que el colchón sea cómodo y la manta no sea pesada. - The important thing is that the mattress is comfortable and the blanket isn’t heavy.

Because lo + adjective turns an adjective into an abstract idea: lo importante = “the important thing/what’s important.” Lo here is the neuter article; it doesn’t mark masculine or feminine. You’d use el/la only if there were an actual noun: e.g., la cosa importante (“the important thing”), el punto importante (“the important point”).

Does importante have to agree in gender/number here?

No. In the lo + adjective pattern, the adjective stays in the default (masculine singular) form regardless of what it refers to: lo bueno, lo malo, lo interesante, lo importante.

Why is the subjunctive sea used after Lo importante es que?

Expressions of evaluation/judgment (e.g., es importante que, es esencial que, lo bueno es que) typically trigger the subjunctive because they present what follows as a desired/required quality rather than a confirmed fact. Hence: Lo importante es que el colchón sea cómodo…

Could I use the indicative (es) instead of the subjunctive (sea)?

It’s not the standard choice. Using the indicative (…es cómodo) would assert that fact as already known/true, which clashes with the evaluative setup and sounds off in most contexts. Stick to the subjunctive after Lo importante es que…

Why isn’t it de que (i.e., Lo importante es de que…)?

Because ser + adjetivo + que takes plain que, not de que. Using de que here is an error (called “dequeísmo”). Some expressions legitimately take de que (e.g., estoy seguro de que, soy consciente de que), but es importante que does not.

Should the second que be repeated before la manta?

It can be, and many speakers prefer it for clarity and rhythm:

Without repetition (your sentence): …que el colchón sea cómodo y la manta no sea pesada.
With repetition: …que el colchón sea cómodo y que la manta no sea pesada.
Both are correct.

Why ser with cómodo for the mattress, not estar?

For inanimate objects, inherent/defining qualities take ser: La cama/el colchón es cómodo. Estar cómodo is used for people (or living beings) feeling comfortable: Estoy cómodo en esta cama.

Why la manta no sea pesada and not la manta esté pesada or sea no pesada?

For physical weight as a characteristic, Spanish uses ser: La manta es pesada/ligera.
Estar pesado tends to mean “to be annoying” (for people) or, with food, “to sit heavy.”
Negation goes before the verb: no sea pesada. You don’t say sea no pesada. A more natural positive alternative is sea ligera (“be light”).

How do the adjectives agree here?

el colchón is masculine singular → cómodo.
la manta is feminine singular → pesada.
If both were plural: los colchones sean cómodos; las mantas no sean pesadas.

Can I avoid the negation by using a positive adjective?

Yes. Two natural options:

…y que la manta sea ligera.
…y que la manta no pese mucho. (Here pese is the present subjunctive of pesar.)

Can I say it without a que-clause?

Yes, as a noun phrase: Lo importante es un colchón cómodo y una manta ligera.
Nuance: the que-clause frames it as a requirement/condition; the noun phrase feels more like listing the key items.

What exact form is sea?

It’s the 3rd person singular present subjunctive of ser. You see it twice, once for each subject: el colchón and la manta.

Why use el/la (the) and not un/una (a)?

Spanish often uses the definite article for things already identified by context or when speaking generically in structures like this. If you want to keep it indefinite, rephrase: Lo importante de un colchón es que sea cómodo.

How would I say this in the past?

Use the imperfect in the main clause and the imperfect subjunctive in the que-clause:
Lo importante era que el colchón fuera (o fuese) cómodo y que la manta no fuera (o fuese) pesada.

Why do colchón and cómodo have accents, but manta, pesada, and sea don’t?

colchón: stress on the last syllable, and it ends in -n → written accent.
cómodo: esdrújula (stress on the antepenultimate syllable) → always accented.
manta, pesada, sea follow regular stress rules and don’t need an accent.

Is manta the right word in Spain? What about other bedding words?

Yes. In Spain:

manta = blanket.
edredón / nórdico = duvet.
Elsewhere you may hear frazada (Cono Sur) or cobija (Mexico/Central America).
Also: colchón = mattress; almohada = pillow; cojín = cushion.

Can I invert the sentence?

Yes: Que el colchón sea cómodo y que la manta no sea pesada es lo importante. This is stylistically marked but correct.

Could I use para que here?

No. Para que expresses purpose (“so that”). Here, que introduces a content clause after an evaluation: Lo importante es que…, not para que….