Learn Italian: Sembra che le mele diventino più mature. - It seems the apples are getting riper.

The phrase sembra che (it seems that) expresses an opinion, doubt, or uncertainty. In Italian, these types of impersonal expressions trigger the subjunctive mood in the clause that follows them.

How is the subjunctive form diventino built for an -are verb like diventare?

In the present subjunctive, regular -are verbs switch their typical -a vowels to -i. For the loro (they) form, the normal ending -ano becomes -ino. That is why diventano changes to diventino.

The translation says 'getting riper', but which word means 'getting'?

Italian doesn't use a direct translation of 'getting' in this context. Instead, it uses the verb diventare, which means 'to become'. So diventino più mature translates literally to 'they become more ripe'.

Why is the adjective mature instead of maturo or maturi?

Adjectives must match the noun they describe in gender and number. Since le mele (the apples) is a feminine plural noun, the adjective takes the feminine plural ending -e, resulting in mature.