No you better don't

[semaphore]

Feasgar math dhuibh,

I've been having difficulty talking about "had better not be/you better not be".

I usually add some Gaelic examples with my questions but this time I'm a little stumped as to what is most natural, so I'll go with English for now.

You better not be telling lies. Better not be lying.
Are you lying? - No! - You better not be!
You better not (do a thing)
Go on, have one! - I better not!

Some of these meanings can be expressed by "don't you dare [... something]" as well. I can express simliar things in Gaelic using "math" or "feum" or idioms with "anam/aghaidh". The primary construction that comes to mind is "s fheàrride / b' fheàrride" but flows naturally for me in the positive "you better/I'd better". I've not had much occasion until now to tell someone they "better not be".

Of course as always depending on context things may go from Gàidhlig gu Beurla a different way. For example

Na faiceam a-rithist sin! - You better not do that again

As usual I can talk around it and that does for the most part, but still feels a bit too indirect. Any help would be great.

[GunChleoc]

Some expressions that should do it:

Cha bu chòir dhut breugan innse.
Na innis breugan!
Ann an da-rìribh?
Na can rium!
Na bi ri...

[MacFhraing]

Maybe it's something using 'gun a bhith' that you want?

"'S fheàirrde thu gun a bhith a' bruidhinn" / "B' fheàirrde mi gun a bhith ag òl"

[semaphore]

Abair gur i seachdain thrang a bh' innte cha robh diog agam gu seo sùil a thoirt air seo. Taing dhuibh airson bhur molaidhean.

'S e "na h-innis" a fheumadh a bhith ann a sin, tha mi creids'. Rinn mi cagnadh air na dh'iarr mi oirbh an t-seachdain-sa is thàinig criomag neo dha thugam. A thaobh an fhir-sa

Go on, have one! - I better not! (I best not) 'S e "chan eil math dhomh" a dh'fhaodadh a bhith na b' fheàrr. Ach eadar 's gum meudaich a' chiall sin 's e comasach a cleachdadh ag ràdh gin dhan fheadhainn eile, 's beag m'fhios.

The meaning of 's fheàirrde is so etched in my brain I can't make it work. I'd need to be tied to a chair and have native speakers scream it at me before I'd accept it. 'S fheàirrde thu gun a bhith a' bruidhinn to me just sounds like "seriously, you're making your situation worse, stop talking" but happy to be wrong on that one.

I can see "na bi thusa ri innse bhreugan" working. The more I think about "better not" the less literal meaning it has, it's more of a preamble to let children know we're threatening them lol. It'd actually be more natural in my Scots-English to say "Don't you be [... telling lies]" than the more (Englishy?Americany?) "You better not be lying". Still, if anything else comes to mind I'll add it as a response to this post.

[GunChleoc]

'S e "na h-innis" a fheumadh a bhith ann a sin, tha mi creids'

That would turn the negative particle "na" into an article or possessive pronoun, and the verb "innis" into a noun. So, that wouldn't make sense

'S fheàirrde thu gun a bhith a' bruidhinn to me just sounds like "seriously, you're making your situation worse, stop talking" but happy to be wrong on that one.

That sounds good to me.

And yes, that construction does take some time getting used to when you are used to Germanic or Latin language patterns. It is very Gaelic if you know what I mean

[akerbeltz]

There's also chan fhuilear dhut sin a dhèanamh "you had better not do that" but beware, the word fuilear is a Gaelic rabbit hole

[GunChleoc]

Negated expressions can be so much fun.

Cha chreid mi nach eil seo feumail!

[semaphore]

That would turn the negative particle "na" into an article or possessive pronoun, and the verb "innis" into a noun. So, that wouldn't make sense

Ah I tend to leave that in as it's what I hear most often. I think it's still there in Irish. Could just be dialectal and probably not sabhal mòr ostaig approved. I believe 'h-abair' for 'abair' is floating about out there somewhere. Ach 's fheàrr leam e.

na h-abair sin
na h-ith mar sin e
na h-òl a-nochd!

Thanks for some direction I can surface the language I need with a bit more confidence. "Fhuilear" doesn't look foreign, it's ringing a bell but is not something I've ever used in anger. Deagh fheasgar dhuibh!

[GunChleoc]

Now you have me checking my grammar books

Michel Byrne: No vowel example
William Lamb: na ithibh sin
Richard Cox: "tha analachadh ro fhuaimreag ion-teasgaidh: na h-ith sin! na ith sin!"

So, you're right and it's optional. I just hadn't come across it yet. Thank you for questioning my statement!