Learn Portuguese (Portugal): É essencial que o mar esteja limpo.

The phrase É essencial que (It is essential that) is an impersonal expression showing valuation or importance. In Portuguese, these expressions trigger the subjunctive mood in the next part of the sentence. That is why we use the present subjunctive esteja instead of the regular present tense está.

Why do we use a form of estar (esteja) instead of ser (seja) for being clean?

We use the verb estar for conditions and temporary states. Whether the sea is clean or dirty is a physical condition that can change, not a permanent, unchangeable characteristic of the water itself. Therefore, we use estar.

How do we conjugate estar in the present subjunctive?

Estar is irregular in the present subjunctive. The forms are: que eu esteja, que tu estejas, que ele/ela/você esteja, que nós estejamos, and que eles/elas/vocês estejam. Since o mar (the sea) is a third-person singular noun, it takes esteja.

Would limpo change if we were talking about a feminine noun, like the beach (a praia)?

Yes! Adjectives must always agree in gender and number with the noun they describe. If we were talking about the beach, the sentence would be É essencial que a praia esteja limpa.