一方だ details
Examples
only keeps happening — a trend intensifying in one direction with no reversal in sight
最近、体重が増える一方だ。
saikin, taijuu ga fueru ippou da.
Lately, my weight just keeps increasing. neutral
給料が上がらなくて、生活費は上がる一方だよ。
kyuuryou ga agaranakute, seikatsuhi wa agaru ippou da yo.
My pay never goes up, but my living costs just keep rising. casual
少子高齢化が進み、労働人口は減る一方です。
shoushikoureika ga susumi, roudoujinkou wa heru ippou desu.
As the population continues to age and have fewer children, the working-age population just keeps shrinking. formal
Formation
増える・減る一方だ（辞書形のみ）attaches to plain clause
Notes
変わる／悪化する…) — most verbs sound odd here. The verb stays dictionary
form even narrating a past trend (上がる一方だった, never ×上がった一方だった).
Related patterns
Similar
ていく tracks any gradual change moving on from now, in whichever direction context supplies, often with an implied endpoint (景色[けしき]が変[か]わっていく); 一方だ narrows to a SINGLE relentless direction with no sign of reversal, most often a worsening trend (悪化[あっか]する一方だ) — swapping ていく back in softens the sense that the trend won't stop.
Lookalikes
一方で sets two parallel situations side by side (北海道は寒い一方で、沖縄は暖かい); 一方だ — no で, verb-dict only — instead tracks a single trend intensifying with no sign of reversal (物価が上がる一方だ) — losing the で turns a two-sided contrast into a one-way trajectory, an easy miscopy since both start 一方.