Call the person, change the word
Family terms take special forms in direct address
What to notice
Compare мама—“mom” as a label or subject—with мамо when speaking to Mom directly. The vocative form marks the person being addressed.
Common family examples include тато → тату, бабуся → бабусю, дідусь → дідусю, сестра → сестро, and брат → брате. Names also have vocative forms.
In writing, direct address is normally separated by commas: Мамо, де мої ключі? In speech, intonation helps listeners hear the address.
The vocative is living everyday grammar, not only poetic language. Learners can practice it first with the people they speak to most.
For beginners, a small set of high-frequency vocatives is more useful than memorizing every ending at once. Repeatedly hearing мамо, тату, бабусю, and a few personal names builds the speaking-about versus speaking-to distinction before the full declension system is introduced.