Learn Turkish: Grubumuz şimdi çalışıyor.

The word for "group" is grup. To make it "our group", we add the first-person plural possessive suffix. Because grup ends in a consonant, the suffix starts with a vowel, becoming -umuz.

Why does grup change to grub- with a 'b'?

This is due to consonant mutation (often called the "k-t-ç-p" rule). When a Turkish word ends in a hard consonant like p, ç, t, or k, and you add a suffix starting with a vowel (like -umuz), that final consonant usually softens. Here, the hard p softens into a b.

Why is the possessive suffix exactly -umuz and not -ımız or -imiz?

This suffix follows 4-way vowel harmony. We look at the last vowel in the root word, which is u in grup. According to the rules of 4-way harmony, if the last vowel is o or u, the vowel in the suffix must also be u.

How is the verb çalışıyor built?