～一方だ— Heading Ever Further In One Direction

• 物価は上がる一方だ。

  Bukka wa agaru ippou da.

  Prices just keep going up.

  上がる (to rise) + 一方だ shows an unbroken upward trend with no sign of reversal.

• 彼の病気は悪くなる一方で、家族は心配している。

  Kare no byouki wa waruku naru ippou de, kazoku wa shinpai shite iru.

  His illness only keeps getting worse, and the family is worried.

  悪くなる (to get worse) + 一方; here 一方で connects to a following clause.

• 田舎の人口は減る一方だ。

  Inaka no jinkou wa heru ippou da.

  The rural population just keeps declining.

  減る (to decrease) + 一方だ expresses a continuous one-directional decline.

• ・Only attaches to the dictionary (plain non-past) form of verbs expressing change.
• ・Often used for negative trends (worsening, rising costs), implying worry.
• ・Do not confuse with 一方(で) meaning 'on the other hand'.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～一方だ is tested against.

• ～がちだ — Prone To It
• ～からいうと／からいえば — Speaking In Terms Of
• ～からして — Even Just From
• ～かわりに — Instead Of / In Exchange
• ～きり — Only / And Never Since
• だらけ — Covered All Over
• っけ — What Was It Again?
• ～っぱなし — Left As It Is