ago

used after a length of time (such as two days, six months, ten years) to say how long before now an event happened.

• before

  'before' marks a point earlier than another past moment (e.g. 'the day before'); 'ago' counts back from now.

• previously

  more formal; doesn't require a measured time phrase and can stand alone.

• back

  informal, often paired with 'ago' or used alone ('a few years back').

• from now

  points forward into the future ('two years from now').

• hence

  formal/literary; means 'from now' ('a week hence').

happening or existing at a point in the past, when describing a stretch of time that has already gone by.

• past

  freely used as a true adjective before nouns ('past summers'); 'ago' cannot.

• bygone

  literary, attributive ('bygone days'); 'ago' is predicative only.