。
She drank a glass of milk.
Literal
[topic-は] milk [object-を] 1-counter-杯 drank.
杯 (はい) is the counter for cups, glasses, and bowlfuls of liquid — 一杯 ('one glass'), 二杯 (two), 三杯 (three, with rendaku to さんばい). The same counter doubles as the noun 'a drink' in expressions like 一杯どう? ('how about a drink?'). The numeric+counter 一杯 here functions as a quantifier modifying the verb directly — Japanese counters often appear without explicit linking, in this 'naked counter' position. Cultural note: a single 'glass of milk' is iconic of children's drinks and simple home scenes — Japanese children commonly take a glass of milk before bed.