Learn Turkish: Çocuklar mağaranın içinde kaybolmamalı. - The children must not get lost inside the cave.

The verb root is kaybol (to get lost). We first add the negative suffix -ma, followed by the obligation suffix -malı (must / should). Due to vowel harmony with the 'o' in the root, we use the 'a' versions of these suffixes. This gives us kaybolmamalı (must not get lost).

Is kaybolmamalı made of two different words?

Yes! It comes from the noun kayıp (lost) and the helper verb olmak (to be/become). When they combine, the 'ı' in kayıp drops out, and the 'p' softens to 'b'. This turns kayıp olmak into the single compound verb kaybolmak (to get lost).

Why does mağara (cave) have the -nın suffix in mağaranın içinde?

To say 'inside the cave' in Turkish, you literally say 'at the cave's inside'. Mağara takes the genitive (possessive) suffix -nın to mean 'the cave's'. The word iç (inside) takes the possessive suffix -i, followed by the buffer letter -n-, and finally the locative suffix -de (at/in).

Since the subject çocuklar is plural, why isn't the verb kaybolmamalılar?