、。
She used to keep a diary, but she doesn't anymore.
Literal
She [topic-は] long-ago, diary [object-を] was-keeping [but-が] now [contrast-は] is-not-keeping.
日記をつける means 'to keep a diary' — つける here has the sense of 'to log' or 'to record.' The sentence contrasts past and present with が ('but'): 昔つけていた ('used to keep') vs 今はつけていない ('now doesn't keep'). The contrastive は after 今 emphasizes the shift: 'now (as opposed to before).' This past-vs-present pattern is a natural way to describe changed habits.