conditionals - "If you be my love” vs. ”If you were my {love}"

I was wondering how somebody in the 19th century would have said or written

1. If you are/were my girl/boyfriend, you must never cheat on me

I thought it might be something like this

2. If you be my love, you shall never be untrue

Halt! Before anyone objects and says that "shall" is reserved for the first person singular and plural, I did check and discovered this Wikipedia entry for Shall and will

Whether or not the above-mentioned prescriptive rule (shall for the unmarked future in the first person) is adhered to, there are certain meanings in which either will or shall tends to be used rather than the other.[…]. However, there are also cases in which the meaning being expressed combines plain futurity with some additional implication; these can be referred to as "coloured" uses of the future markers.

Thus shall may be used (particularly in the second and third persons) to imply a command, promise or threat made by the speaker (i.e. that the future event denoted represents the will of the speaker rather than that of the subject). For example:

• You shall regret it before long. (speaker's threat)


• You shall not pass! (speaker's command)


• You shall go to the ball. (speaker's promise)

Questions

1. Is my 19th-century version realistic? Did I miss out on something?





2. What type of conditional is sentence number 2 called?

Answer

Your first sentence could not be in the 19th c. as it includes words and expressions that were not used then. Such as 'girlfriend' and 'cheat on'.

I looked up 19th c. songs, because I thought they'd be a good source of 19th c. popular language. And also because I'm a songwriter of very old songs, and I love them!

Well 'girlfriend' wouldn't be there - that's a modern word from c. 1922. Nor would 'sugar' be an option - that's from the 30's.
19th c. versions might be 'mistress' - which just meant 'young woman', 'master' which meant 'man or young man'. Or 'my love' or 'darling'. Or 'sweetheart'. Or 'my dear'. 'Wife', or 'maiden' meaning 'young girl' could be there as well.

Sweetheart:
https://www.sheetmusicwarehouse.co.uk/19th-century-songs-m/my-sweetheart-when-a-boy-song-in-the-key-of-e-flat-major-for-higher-voice/

'Cheat on me' wouldn't be there, either, in the sense of 'cheating in love', as that's from the 1930s.
'False': The 19th c way of saying that might be that you, or your love was 'false'.

Origin of 'Cheat in love': https://psmag.com/social-justice/cheaters-history-cheating-68591

In the 19th c. to convey 'ever', you'd simply use the archaic 'ere'
It means '(if) ever you were'. So it implies possibility, eg:

Ere you are my love, you must ne'er be untrue

Actually there is 'ere' - which means 'before' - 'ere you were my love' which is archaic
Then there is "e'er" which means 'ever'
But 'ere' seems more natural to me, here.

Ere: http://www.dictionary.com/browse/ere

In the 19th c. 'never' is often contracted, written "ne'er" with an apostrope for the missing 'v'. This is common in songs. It sounds like 'air' as in 'ne'er do well' which is probably familiar to you. You can also find 'nairy' in old songs, which means 'never', or 'not any' as well. 'Nairy a day goes by when I don't think of you'.

Nairy: https://www.etymonline.com/word/never
Poetic contraction (various): https://en.m.wikipedia.org/wiki/Poetic_contraction

So to return to your question, here are some 19th c. examples of how you might say it:

• 'Ere you are my darling, you must nairy be untrue'


• 'Ere you are my love, you'll ne'er be false to me'


• 'If you be my love, you shall ne'er be untrue'



• 'If you be my dear, you'll ne'er be false to me'