"A Poem of Changgan" is narrated by a girl who becomes a wife. It's a very personal poem, about how sad the speaker feel about being separated from her husband, who's gone away on a long journey. One way that the speaker gives us an insight into her internal state is through descriptions of her physical development, how her face and body change over the course of time. She starts off as teeny-weeny girl and then grows up, and each stage of her development is reflected in her physical appearance.
Line 1: By telling us that her "hair had hardly covered [her] forehead" in the first line, the speaker clues us into the fact that she's still just a tiny little girl when she first meets her future husband. When we're very little, our hair takes a while to grow out. So this is a way that the speaker tells us that she's a child.
Line 8: At fourteen, when the speaker marries her sweetheart, she's "[s]o bashful that I dared not smile." All teenagers are awkward at that age, aren't they? And we'd be especially awkward if we had to get married at fourteen, that's for sure. That's pretty young, but hey, that's how people did things back in the day. Again, by giving us a description of her physical appearance and behavior here, the speaker gives us an insight into her emotional development, too.
Line 11: At fifteen the speaker is a very different person, "straightening" her brows and laughing. No, that doesn't mean she tweezes her eyebrows. It means that she isn't frowning all the time because she's so shy. She's laughing now. The physical description of the speaker here shows us how she has developed and matured, embracing her relationship with her husband.
Line 26: By saying that she "fears for [her] bright cheeks, lest they fade," the speaker gives us an indication of her sadness. Her sweetheart's been gone for months, and it's taking a toll on her. Again, through giving us a description of the physical changes she's undergoing, the speaker gives us an insight into her emotional state.