Former is used of time, or of position in written or printed matter, not of space in general. We can say former times, a former chapter, etc., but not the former part of a garden; we should say the front part of the garden, the forward car of a train.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

back You shouldn't talk back to your parents like that.
old You are not old enough to go swimming by yourself.
historical The Byzantine Empire is a historical construction.

Popularity over time

Word	1850 → 2019	Trend
former		steady
back		rising
whilom		steady
old		rising
historical		rising

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: old¹
4 letters: back¹, past¹, late¹, fore¹
5 letters: other², early², prior², olden²
6 letters: bygone², whilom
7 letters: earlier³, ancient², quondam
8 letters: previous³, anterior⁴, foregone²
9 letters: precedent³, preceding³, foregoing³, erstwhile²
10 letters: historical⁴, antecedent⁴

The small number after each word is its number of syllables (other² = 2 syllables).