Learn French: Je suis content que la marée soit basse.

Because the sentence starts with Je suis content que (I am happy that). In French, expressing an emotion or feeling about someone or something else requires the verb in the following clause to be in the subjunctive mood. Soit is the subjunctive form of être (to be).

How do I conjugate être in the subjunctive?

Être is entirely irregular in the subjunctive. The forms are: que je sois, que tu sois, qu'il/elle soit, que nous soyons, que vous soyez, and qu'ils/elles soient. Since la marée (the tide) is singular, it is equivalent to elle, so we use soit.

Why is the adjective spelled basse instead of bas?

Adjectives must agree in gender and number with the noun they describe. Because la marée is a feminine singular noun, the masculine adjective bas (low) doubles its final consonant and adds an -e to become the feminine form basse.

Could I use de instead of que here, like Je suis content d'être...?