Learn Hardcore Spanish (Latin America): La cosa importante es mantener la casa limpia. - The important thing is to keep the house clean.

In Spanish, importante remains the same in both singular and plural forms for masculine and feminine nouns. Here, it modifies cosa, which is singular, so importante is used in its singular form.

Why is mantener in the infinitive form?

After the verb es, we often use an infinitive to express a general activity or concept. In this case, mantener acts like a noun phrase describing what the important thing is (i.e., the important thing is to maintain).

Why do we say la casa limpia instead of la casa limpio?

Because casa is a feminine noun in Spanish, the adjective must agree in gender. Limpia is the feminine form, so it matches la casa.

Can we say lo importante instead of la cosa importante?

Yes. Lo importante literally means what is important, and it is commonly used to talk about the essential point or matter. La cosa importante is just a slightly more explicit way of saying the important thing.