〜一方だ
keeps (getting more / less), only ever
Verb dictionary form + 一方だ
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一方だ describes a trend that keeps going in one direction: 物価は上がる一方だ, "prices just keep going up". 一方 means "one way", so it's literally "only the one way".
It goes after the dictionary form of verbs of change: 増える, 減る, 上がる, 下がる, 悪くなる, 大きくなる. The trend is usually unwelcome, and the sentence often carries a complaint or worry.
In the past, 一方だった describes a trend that kept going: 雨は強くなる一方だった, "the rain just kept getting heavier".
Don't confuse it with 一方で, "on the other hand", which joins two contrasting facts. That comes later in this deck.
Examples
物価は上がる一方だ。
Prices just keep going up.
最近、仕事が増える一方です。
Lately my workload just keeps growing.
町の人口は減る一方だ。
The town's population keeps shrinking.
雨は強くなる一方だった。
The rain just kept getting heavier.
このままでは、問題は大きくなる一方だ。
At this rate, the problem will only get bigger.