Then is used to talk about what comes next in time or order, as in 'First we ate, then we left.' It can also refer to a particular time in the past or future, as in 'by then' or 'since then'.
From Old English *þanne, þænne* 'at that time, then', related to *þan* ('than'). Originally, it covered many time and comparison uses that later split into 'then' and 'than'.
English once used almost the same word for 'than' and 'then', which is why learners often confuse them. 'Then' is your time anchor—whenever you see it, something is being placed on a timeline.
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