一方だ expresses that something is continuously increasing or progressing in a single direction, often with a negative nuance. It is used for trends that are gradually getting worse or more intense over time, like rising prices, worsening health, or increasing crime. You cannot use it for temporary changes or for changes that might reverse soon.

This pattern is mostly used in written or formal spoken Japanese, such as news reports or essays. In casual conversation, you'd more likely hear どんどん～ていく or ばかりだ. Because it describes a one-way trend, it doesn't work with verbs that can go back and forth, like 変わる (to change) – you wouldn't say 変わる一方だ unless the change is consistently one-directional. Stick to verbs that clearly indicate an increase or decrease.

The ending is だ in plain form; in polite speech, it becomes です. So you might say 増える一方です in a formal conversation. The pattern is not rude, but it sounds a bit objective and detached, like reporting facts.