Learn Arabic (Modern Standard): الماء غال في الصحراء.

Words like غالي belong to a special group of words (defective nouns) ending in a 'yaa' (ي). When these words are indefinite (they don't have ال) and are in the nominative or genitive case, they drop that final ي. Because this word is the predicate of the sentence, it is in the nominative case, so غالي loses its last letter and is written as غال.

Since غال takes a double kasra (ghālin), does that mean it is in the genitive case?

Surprisingly, no! It is in the nominative case because it is the predicate of the sentence. The double kasra here is not a case marker. Instead, it acts as a spelling placeholder to show that the final ي has been dropped from the original word.

In English we just say 'Water' without 'The', but the Arabic uses the definite article الماء. Why?

In Arabic, when you are making a broad statement about a general category, a substance, or an abstract concept (like water, love, or time), you must use the definite article ال (al-). Using an indefinite noun here would sound strangely specific, almost like saying 'A water is expensive'.

The word الصحراء (the desert) is treated as feminine, but it doesn't end in a taa marbuuta (ة). How do we know it is feminine?